[PATCH] usb: usbfs: correct kernel->user page attribute mismatch

[PATCH] usb: usbfs: correct kernel->user page attribute mismatch

[Jeremy Linton]

On arm64, and possibly other architectures, requesting
IO coherent memory may return Normal-NC if the underlying
hardware isn't coherent. If these pages are then
remapped into userspace as Normal, that defeats the
purpose of getting Normal-NC, as well as resulting in
mappings with differing cache attributes.

In particular this happens with libusb, when it attempts
to create zero-copy buffers as is used by rtl-sdr, and
maybe other applications. The result is usually
application death.

If dma_mmap_attr() is used instead of remap_pfn_range,
the page cache/etc attributes can be matched between the
kernel and userspace.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
---
 drivers/usb/core/devio.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
index 6833c918abce..1e7458dd6e5d 100644
--- a/drivers/usb/core/devio.c
+++ b/drivers/usb/core/devio.c
@@ -217,6 +217,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
 {
 	struct usb_memory *usbm = NULL;
 	struct usb_dev_state *ps = file->private_data;
+	struct usb_hcd *hcd = bus_to_hcd(ps->dev->bus);
 	size_t size = vma->vm_end - vma->vm_start;
 	void *mem;
 	unsigned long flags;
@@ -250,9 +251,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
 	usbm->vma_use_count = 1;
 	INIT_LIST_HEAD(&usbm->memlist);
 
-	if (remap_pfn_range(vma, vma->vm_start,
-			virt_to_phys(usbm->mem) >> PAGE_SHIFT,
-			size, vma->vm_page_prot) < 0) {
+	if (dma_mmap_attrs(hcd->self.sysdev, vma, mem, dma_handle, size, 0)) {
 		dec_usb_memory_use_count(usbm, &usbm->vma_use_count);
 		return -EAGAIN;
 	}
-- 
2.24.1

Re: [PATCH] usb: usbfs: correct kernel->user page attribute mismatch

[Greg KH]

On Thu, Apr 30, 2020 at 04:19:22PM -0500, Jeremy Linton wrote:
> On arm64, and possibly other architectures, requesting
> IO coherent memory may return Normal-NC if the underlying
> hardware isn't coherent. If these pages are then
> remapped into userspace as Normal, that defeats the
> purpose of getting Normal-NC, as well as resulting in
> mappings with differing cache attributes.

What is "Normal-NC"?

> In particular this happens with libusb, when it attempts
> to create zero-copy buffers as is used by rtl-sdr, and

What is "rtl-sdr"

> maybe other applications. The result is usually
> application death.

So is this a new problem?  Old problem?  Old problem only showing up on
future devices?  On current devices?  I need a hint here as to know if
this is a bugfix or just work to make future devices work properly.

> 
> If dma_mmap_attr() is used instead of remap_pfn_range,
> the page cache/etc attributes can be matched between the
> kernel and userspace.
> 
> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
> ---
>  drivers/usb/core/devio.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
> index 6833c918abce..1e7458dd6e5d 100644
> --- a/drivers/usb/core/devio.c
> +++ b/drivers/usb/core/devio.c
> @@ -217,6 +217,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
>  {
>  	struct usb_memory *usbm = NULL;
>  	struct usb_dev_state *ps = file->private_data;
> +	struct usb_hcd *hcd = bus_to_hcd(ps->dev->bus);
>  	size_t size = vma->vm_end - vma->vm_start;
>  	void *mem;
>  	unsigned long flags;
> @@ -250,9 +251,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
>  	usbm->vma_use_count = 1;
>  	INIT_LIST_HEAD(&usbm->memlist);
>  
> -	if (remap_pfn_range(vma, vma->vm_start,
> -			virt_to_phys(usbm->mem) >> PAGE_SHIFT,
> -			size, vma->vm_page_prot) < 0) {
> +	if (dma_mmap_attrs(hcd->self.sysdev, vma, mem, dma_handle, size, 0)) {

Given that this code has not changed since 2016, how has no one noticed
this issue before?

And have you tested this change out on other systems (i.e. x86) to
ensure that this still works properly?

And why isn't this call used more by drivers if this is a real issue?
And will this cause issues with how the userspace mapping is handled as
now we rely on userspace to do things differently?  Or am I reading the
dma_mmap_attrs() documentation wrong?

thanks,

greg k-h

Re: [PATCH] usb: usbfs: correct kernel->user page attribute mismatch

[Mark Rutland]

On Fri, May 01, 2020 at 09:05:00AM +0200, Greg KH wrote:
> On Thu, Apr 30, 2020 at 04:19:22PM -0500, Jeremy Linton wrote:
> > On arm64, and possibly other architectures, requesting
> > IO coherent memory may return Normal-NC if the underlying
> > hardware isn't coherent. If these pages are then
> > remapped into userspace as Normal, that defeats the
> > purpose of getting Normal-NC, as well as resulting in
> > mappings with differing cache attributes.
> 
> What is "Normal-NC"?

Arm terminology for "Normal Non-Cacheable"; it might be better to say
something like:

On some architectures (e.g. arm64) an IO coherent mapping may use
non-cachable attributes if the relevant device is cache coherent.
If userspace mappings are cacheable, these may not be coherent with
non-cacheable mappings. On arm64 this is the case for Normal-NC and
Normal (cacheable) mappings.

Thanks,
Mark.

Re: [PATCH] usb: usbfs: correct kernel->user page attribute mismatch

[Marc Zyngier]

On 2020-05-01 11:37, Mark Rutland wrote:
> On Fri, May 01, 2020 at 09:05:00AM +0200, Greg KH wrote:
>> On Thu, Apr 30, 2020 at 04:19:22PM -0500, Jeremy Linton wrote:
>> > On arm64, and possibly other architectures, requesting
>> > IO coherent memory may return Normal-NC if the underlying
>> > hardware isn't coherent. If these pages are then
>> > remapped into userspace as Normal, that defeats the
>> > purpose of getting Normal-NC, as well as resulting in
>> > mappings with differing cache attributes.
>> 
>> What is "Normal-NC"?
> 
> Arm terminology for "Normal Non-Cacheable"; it might be better to say
> something like:
> 
> On some architectures (e.g. arm64) an IO coherent mapping may use
> non-cachable attributes if the relevant device is cache coherent.

is *not* cache coherent.

> If userspace mappings are cacheable, these may not be coherent with
> non-cacheable mappings. On arm64 this is the case for Normal-NC and
> Normal (cacheable) mappings.

And to answer some of Greg's questions:

- This can show up on current HW that doesn't offer full IO coherency,
   which is likely on low-end arm/arm64 systems.

- I guess nobody noticed this before as x86 is perfectly happy with
   conflicting attributes for the same physical page, and there is
   (wild guess) probably not that much userspace making use of shared
   mappings between kernel and userspace using this interface.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...

Re: [PATCH] usb: usbfs: correct kernel->user page attribute mismatch

[Greg KH]

On Fri, May 01, 2020 at 11:37:12AM +0100, Mark Rutland wrote:
> On Fri, May 01, 2020 at 09:05:00AM +0200, Greg KH wrote:
> > On Thu, Apr 30, 2020 at 04:19:22PM -0500, Jeremy Linton wrote:
> > > On arm64, and possibly other architectures, requesting
> > > IO coherent memory may return Normal-NC if the underlying
> > > hardware isn't coherent. If these pages are then
> > > remapped into userspace as Normal, that defeats the
> > > purpose of getting Normal-NC, as well as resulting in
> > > mappings with differing cache attributes.
> > 
> > What is "Normal-NC"?
> 
> Arm terminology for "Normal Non-Cacheable"; it might be better to say
> something like:
> 
> On some architectures (e.g. arm64) an IO coherent mapping may use
> non-cachable attributes if the relevant device is cache coherent.
> If userspace mappings are cacheable, these may not be coherent with
> non-cacheable mappings. On arm64 this is the case for Normal-NC and
> Normal (cacheable) mappings.

That's better, but it doesn't answer any of my other questions on this
patch :)

thanks,

greg k-h

Re: [PATCH] usb: usbfs: correct kernel->user page attribute mismatch

[Robin Murphy]

On 2020-05-01 8:05 am, Greg KH wrote:
> On Thu, Apr 30, 2020 at 04:19:22PM -0500, Jeremy Linton wrote:
>> On arm64, and possibly other architectures, requesting
>> IO coherent memory may return Normal-NC if the underlying
>> hardware isn't coherent. If these pages are then
>> remapped into userspace as Normal, that defeats the
>> purpose of getting Normal-NC, as well as resulting in
>> mappings with differing cache attributes.
> 
> What is "Normal-NC"?
> 
>> In particular this happens with libusb, when it attempts
>> to create zero-copy buffers as is used by rtl-sdr, and
> 
> What is "rtl-sdr"
> 
>> maybe other applications. The result is usually
>> application death.
> 
> So is this a new problem?  Old problem?  Old problem only showing up on
> future devices?  On current devices?  I need a hint here as to know if
> this is a bugfix or just work to make future devices work properly.
> 
>>
>> If dma_mmap_attr() is used instead of remap_pfn_range,
>> the page cache/etc attributes can be matched between the
>> kernel and userspace.
>>
>> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
>> ---
>>   drivers/usb/core/devio.c | 5 ++---
>>   1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
>> index 6833c918abce..1e7458dd6e5d 100644
>> --- a/drivers/usb/core/devio.c
>> +++ b/drivers/usb/core/devio.c
>> @@ -217,6 +217,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
>>   {
>>   	struct usb_memory *usbm = NULL;
>>   	struct usb_dev_state *ps = file->private_data;
>> +	struct usb_hcd *hcd = bus_to_hcd(ps->dev->bus);
>>   	size_t size = vma->vm_end - vma->vm_start;
>>   	void *mem;
>>   	unsigned long flags;
>> @@ -250,9 +251,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
>>   	usbm->vma_use_count = 1;
>>   	INIT_LIST_HEAD(&usbm->memlist);
>>   
>> -	if (remap_pfn_range(vma, vma->vm_start,
>> -			virt_to_phys(usbm->mem) >> PAGE_SHIFT,
>> -			size, vma->vm_page_prot) < 0) {
>> +	if (dma_mmap_attrs(hcd->self.sysdev, vma, mem, dma_handle, size, 0)) {
> 
> Given that this code has not changed since 2016, how has no one noticed
> this issue before?

They have. Here's where the most recent one in my inbox ended, which has 
breadcrumbs to a couple more:

https://lore.kernel.org/linux-arm-kernel/20190808130525.GA1756@kroah.com/

Note the author ;)

 From memory, all the previous attempts wound up getting stuck on the 
subtlety that buffers from hcd_alloc() may or may not actually have come 
from the DMA API. Since then, the localmem_pool rework has probably 
helped a bit, but I'm not sure we've ever really nailed down whether 
kmalloc()ed buffers from PIO-mode controllers (i.e. the !hcd_uses_dma() 
case) can end up down this devio path.

Robin.

Re: [PATCH] usb: usbfs: correct kernel->user page attribute mismatch

[Jeremy Linton]

Hi,

Thanks for taking a look at this.

On 5/1/20 2:05 AM, Greg KH wrote:
> On Thu, Apr 30, 2020 at 04:19:22PM -0500, Jeremy Linton wrote:
>> On arm64, and possibly other architectures, requesting
>> IO coherent memory may return Normal-NC if the underlying
>> hardware isn't coherent. If these pages are then
>> remapped into userspace as Normal, that defeats the
>> purpose of getting Normal-NC, as well as resulting in
>> mappings with differing cache attributes.
> 
> What is "Normal-NC"?

A non-cacheable attribute on arm64 pages. I think Mark R & Marc Z 
elaborated while I was asleep (thanks!).
   .


> 
>> In particular this happens with libusb, when it attempts
>> to create zero-copy buffers as is used by rtl-sdr, and
> 
> What is "rtl-sdr"

Its the realtek software defined radio (SDR), a really inexpensive TV 
dongle that was discovered could be used as a general purpose SDR a 
decade or so ago. In particular, this project
https://github.com/osmocom/rtl-sdr/
which is packaged by fedora/etc.

> 
>> maybe other applications. The result is usually
>> application death.
> 
> So is this a new problem?  Old problem?  Old problem only showing up on
> future devices?  On current devices?  I need a hint here as to know if
> this is a bugfix or just work to make future devices work properly.

This has been a problem on arm devices without IO coherent USB 
apparently for years. The rtl-sdr project itself has a disable zero-copy 
mode that people have been using on rpi/etc specific builds. Fedora 
OTOH, is building it with the same flags on x86 & arm64 which means that 
people report problems. This happened a few days ago (on a pinebook), 
and I duplicated it on an NXP platform just running the `rtl_test` 
artifact with a nooelec from my junk box. Guessing that it was a page 
mismatch I went looking for that, rather than disabling the zero copy 
since punishing arm machine that have IO coherent USB adapters for the 
sins of these low end devices isn't ideal. I found this, and this patch 
allows the rtl_test app to run without issues on my NXP/solidrun.

Plus, given that its actually a kernel/libusb problem its likely there 
are other applications having similar problems.

> 
>>
>> If dma_mmap_attr() is used instead of remap_pfn_range,
>> the page cache/etc attributes can be matched between the
>> kernel and userspace.
>>
>> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
>> ---
>>   drivers/usb/core/devio.c | 5 ++---
>>   1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
>> index 6833c918abce..1e7458dd6e5d 100644
>> --- a/drivers/usb/core/devio.c
>> +++ b/drivers/usb/core/devio.c
>> @@ -217,6 +217,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
>>   {
>>   	struct usb_memory *usbm = NULL;
>>   	struct usb_dev_state *ps = file->private_data;
>> +	struct usb_hcd *hcd = bus_to_hcd(ps->dev->bus);
>>   	size_t size = vma->vm_end - vma->vm_start;
>>   	void *mem;
>>   	unsigned long flags;
>> @@ -250,9 +251,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
>>   	usbm->vma_use_count = 1;
>>   	INIT_LIST_HEAD(&usbm->memlist);
>>   
>> -	if (remap_pfn_range(vma, vma->vm_start,
>> -			virt_to_phys(usbm->mem) >> PAGE_SHIFT,
>> -			size, vma->vm_page_prot) < 0) {
>> +	if (dma_mmap_attrs(hcd->self.sysdev, vma, mem, dma_handle, size, 0)) {
> 
> Given that this code has not changed since 2016, how has no one noticed
> this issue before?
They have there are a lot of reports of sdr failures, but the general 
use case is rare?

> 
> And have you tested this change out on other systems (i.e. x86) to
> ensure that this still works properly?

Yes and no, I did some basic libusb tests on an x86 machine, but its a 
bit tricky at the moment for me to get the rtl plugged into a x86 test 
machine. (its a work in progress).


> 
> And why isn't this call used more by drivers if this is a real issue?
The particulars of asking for iocoherent memory and then mapping it to 
userspace is rarer than just asking for kmalloc()/remap() and then 
performing the dma ops?

Then there are all the softer issues around arm64 testing/availability 
and vendors carrying "fixes" for particular issues (like rtl-sdr 
disabling zero copy).

> And will this cause issues with how the userspace mapping is handled as
> now we rely on userspace to do things differently?  Or am I reading the
> dma_mmap_attrs() documentation wrong?
I don't think userspace is doing anything differently here, and AFAIK, 
on systems with IO coherent adapters this ends up with the same page 
mapping as just doing the remap_pfn_rage() with the same attributes as 
before. I've looked at dma_map_attrs() a bit, but i'm also trusting it 
does what it says on the tin.


Thanks again for looking at this.

> 
> thanks,
> 
> greg k-h
> 

Re: [PATCH] usb: usbfs: correct kernel->user page attribute mismatch

[Greg KH]

On Fri, May 01, 2020 at 10:47:22AM -0500, Jeremy Linton wrote:
> Hi,
> 
> Thanks for taking a look at this.
> 
> On 5/1/20 2:05 AM, Greg KH wrote:
> > On Thu, Apr 30, 2020 at 04:19:22PM -0500, Jeremy Linton wrote:
> > > On arm64, and possibly other architectures, requesting
> > > IO coherent memory may return Normal-NC if the underlying
> > > hardware isn't coherent. If these pages are then
> > > remapped into userspace as Normal, that defeats the
> > > purpose of getting Normal-NC, as well as resulting in
> > > mappings with differing cache attributes.
> > 
> > What is "Normal-NC"?
> 
> A non-cacheable attribute on arm64 pages. I think Mark R & Marc Z elaborated
> while I was asleep (thanks!).
>   .
> 
> 
> > 
> > > In particular this happens with libusb, when it attempts
> > > to create zero-copy buffers as is used by rtl-sdr, and
> > 
> > What is "rtl-sdr"
> 
> Its the realtek software defined radio (SDR), a really inexpensive TV dongle
> that was discovered could be used as a general purpose SDR a decade or so
> ago. In particular, this project
> https://github.com/osmocom/rtl-sdr/
> which is packaged by fedora/etc.
> 
> > 
> > > maybe other applications. The result is usually
> > > application death.
> > 
> > So is this a new problem?  Old problem?  Old problem only showing up on
> > future devices?  On current devices?  I need a hint here as to know if
> > this is a bugfix or just work to make future devices work properly.
> 
> This has been a problem on arm devices without IO coherent USB apparently
> for years. The rtl-sdr project itself has a disable zero-copy mode that
> people have been using on rpi/etc specific builds. Fedora OTOH, is building
> it with the same flags on x86 & arm64 which means that people report
> problems. This happened a few days ago (on a pinebook), and I duplicated it
> on an NXP platform just running the `rtl_test` artifact with a nooelec from
> my junk box. Guessing that it was a page mismatch I went looking for that,
> rather than disabling the zero copy since punishing arm machine that have IO
> coherent USB adapters for the sins of these low end devices isn't ideal. I
> found this, and this patch allows the rtl_test app to run without issues on
> my NXP/solidrun.
> 
> Plus, given that its actually a kernel/libusb problem its likely there are
> other applications having similar problems.
> 
> > 
> > > 
> > > If dma_mmap_attr() is used instead of remap_pfn_range,
> > > the page cache/etc attributes can be matched between the
> > > kernel and userspace.
> > > 
> > > Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
> > > ---
> > >   drivers/usb/core/devio.c | 5 ++---
> > >   1 file changed, 2 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
> > > index 6833c918abce..1e7458dd6e5d 100644
> > > --- a/drivers/usb/core/devio.c
> > > +++ b/drivers/usb/core/devio.c
> > > @@ -217,6 +217,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
> > >   {
> > >   	struct usb_memory *usbm = NULL;
> > >   	struct usb_dev_state *ps = file->private_data;
> > > +	struct usb_hcd *hcd = bus_to_hcd(ps->dev->bus);
> > >   	size_t size = vma->vm_end - vma->vm_start;
> > >   	void *mem;
> > >   	unsigned long flags;
> > > @@ -250,9 +251,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
> > >   	usbm->vma_use_count = 1;
> > >   	INIT_LIST_HEAD(&usbm->memlist);
> > > -	if (remap_pfn_range(vma, vma->vm_start,
> > > -			virt_to_phys(usbm->mem) >> PAGE_SHIFT,
> > > -			size, vma->vm_page_prot) < 0) {
> > > +	if (dma_mmap_attrs(hcd->self.sysdev, vma, mem, dma_handle, size, 0)) {
> > 
> > Given that this code has not changed since 2016, how has no one noticed
> > this issue before?
> They have there are a lot of reports of sdr failures, but the general use
> case is rare?
> 
> > 
> > And have you tested this change out on other systems (i.e. x86) to
> > ensure that this still works properly?
> 
> Yes and no, I did some basic libusb tests on an x86 machine, but its a bit
> tricky at the moment for me to get the rtl plugged into a x86 test machine.
> (its a work in progress).
> 
> 
> > 
> > And why isn't this call used more by drivers if this is a real issue?
> The particulars of asking for iocoherent memory and then mapping it to
> userspace is rarer than just asking for kmalloc()/remap() and then
> performing the dma ops?
> 
> Then there are all the softer issues around arm64 testing/availability and
> vendors carrying "fixes" for particular issues (like rtl-sdr disabling zero
> copy).
> 
> > And will this cause issues with how the userspace mapping is handled as
> > now we rely on userspace to do things differently?  Or am I reading the
> > dma_mmap_attrs() documentation wrong?
> I don't think userspace is doing anything differently here, and AFAIK, on
> systems with IO coherent adapters this ends up with the same page mapping as
> just doing the remap_pfn_rage() with the same attributes as before. I've
> looked at dma_map_attrs() a bit, but i'm also trusting it does what it says
> on the tin.
> 
> 
> Thanks again for looking at this.

Ok, can I get a lot better written changelog text for this patch based
on this thread, so that it makes more sense when we merge this patch?

thanks,

greg k-h

Re: [PATCH] usb: usbfs: correct kernel->user page attribute mismatch

[Jeremy Linton]

Hi,

On 5/4/20 2:13 AM, Greg KH wrote:
> On Fri, May 01, 2020 at 10:47:22AM -0500, Jeremy Linton wrote:
>> Hi,
>>
>> Thanks for taking a look at this.
>>
>> On 5/1/20 2:05 AM, Greg KH wrote:
>>> On Thu, Apr 30, 2020 at 04:19:22PM -0500, Jeremy Linton wrote:
>>>> On arm64, and possibly other architectures, requesting
>>>> IO coherent memory may return Normal-NC if the underlying
>>>> hardware isn't coherent. If these pages are then
>>>> remapped into userspace as Normal, that defeats the
>>>> purpose of getting Normal-NC, as well as resulting in
>>>> mappings with differing cache attributes.
>>>
>>> What is "Normal-NC"?
>>
>> A non-cacheable attribute on arm64 pages. I think Mark R & Marc Z elaborated
>> while I was asleep (thanks!).
>>    .
>>
>>
>>>
>>>> In particular this happens with libusb, when it attempts
>>>> to create zero-copy buffers as is used by rtl-sdr, and
>>>
>>> What is "rtl-sdr"
>>
>> Its the realtek software defined radio (SDR), a really inexpensive TV dongle
>> that was discovered could be used as a general purpose SDR a decade or so
>> ago. In particular, this project
>> https://github.com/osmocom/rtl-sdr/
>> which is packaged by fedora/etc.
>>
>>>
>>>> maybe other applications. The result is usually
>>>> application death.
>>>
>>> So is this a new problem?  Old problem?  Old problem only showing up on
>>> future devices?  On current devices?  I need a hint here as to know if
>>> this is a bugfix or just work to make future devices work properly.
>>
>> This has been a problem on arm devices without IO coherent USB apparently
>> for years. The rtl-sdr project itself has a disable zero-copy mode that
>> people have been using on rpi/etc specific builds. Fedora OTOH, is building
>> it with the same flags on x86 & arm64 which means that people report
>> problems. This happened a few days ago (on a pinebook), and I duplicated it
>> on an NXP platform just running the `rtl_test` artifact with a nooelec from
>> my junk box. Guessing that it was a page mismatch I went looking for that,
>> rather than disabling the zero copy since punishing arm machine that have IO
>> coherent USB adapters for the sins of these low end devices isn't ideal. I
>> found this, and this patch allows the rtl_test app to run without issues on
>> my NXP/solidrun.
>>
>> Plus, given that its actually a kernel/libusb problem its likely there are
>> other applications having similar problems.
>>
>>>
>>>>
>>>> If dma_mmap_attr() is used instead of remap_pfn_range,
>>>> the page cache/etc attributes can be matched between the
>>>> kernel and userspace.
>>>>
>>>> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
>>>> ---
>>>>    drivers/usb/core/devio.c | 5 ++---
>>>>    1 file changed, 2 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
>>>> index 6833c918abce..1e7458dd6e5d 100644
>>>> --- a/drivers/usb/core/devio.c
>>>> +++ b/drivers/usb/core/devio.c
>>>> @@ -217,6 +217,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
>>>>    {
>>>>    	struct usb_memory *usbm = NULL;
>>>>    	struct usb_dev_state *ps = file->private_data;
>>>> +	struct usb_hcd *hcd = bus_to_hcd(ps->dev->bus);
>>>>    	size_t size = vma->vm_end - vma->vm_start;
>>>>    	void *mem;
>>>>    	unsigned long flags;
>>>> @@ -250,9 +251,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
>>>>    	usbm->vma_use_count = 1;
>>>>    	INIT_LIST_HEAD(&usbm->memlist);
>>>> -	if (remap_pfn_range(vma, vma->vm_start,
>>>> -			virt_to_phys(usbm->mem) >> PAGE_SHIFT,
>>>> -			size, vma->vm_page_prot) < 0) {
>>>> +	if (dma_mmap_attrs(hcd->self.sysdev, vma, mem, dma_handle, size, 0)) {
>>>
>>> Given that this code has not changed since 2016, how has no one noticed
>>> this issue before?
>> They have there are a lot of reports of sdr failures, but the general use
>> case is rare?
>>
>>>
>>> And have you tested this change out on other systems (i.e. x86) to
>>> ensure that this still works properly?
>>
>> Yes and no, I did some basic libusb tests on an x86 machine, but its a bit
>> tricky at the moment for me to get the rtl plugged into a x86 test machine.
>> (its a work in progress).
>>
>>
>>>
>>> And why isn't this call used more by drivers if this is a real issue?
>> The particulars of asking for iocoherent memory and then mapping it to
>> userspace is rarer than just asking for kmalloc()/remap() and then
>> performing the dma ops?
>>
>> Then there are all the softer issues around arm64 testing/availability and
>> vendors carrying "fixes" for particular issues (like rtl-sdr disabling zero
>> copy).
>>
>>> And will this cause issues with how the userspace mapping is handled as
>>> now we rely on userspace to do things differently?  Or am I reading the
>>> dma_mmap_attrs() documentation wrong?
>> I don't think userspace is doing anything differently here, and AFAIK, on
>> systems with IO coherent adapters this ends up with the same page mapping as
>> just doing the remap_pfn_rage() with the same attributes as before. I've
>> looked at dma_map_attrs() a bit, but i'm also trusting it does what it says
>> on the tin.
>>
>>
>> Thanks again for looking at this.
> 
> Ok, can I get a lot better written changelog text for this patch based
> on this thread, so that it makes more sense when we merge this patch?

Yes, sure. I also plan on changing it to dma_mmap_coherent() which is 
the same as the dma_mmap_attrs() with the attrs as above.


I will re-post later this afternoon.

Thanks,