[PATCH] usb: dwc3: gadget: Update chain bit correctly when using sg list

[PATCH] usb: dwc3: gadget: Update chain bit correctly when using sg list

[John Stultz]

From: Pratham Pratap <prathampratap@codeaurora.org>

If scatter-gather operation is allowed, a large USB request is split
into multiple TRBs. For preparing TRBs for sg list, driver iterates
over the list and creates TRB for each sg and mark the chain bit to
false for the last sg. The current IOMMU driver is clubbing the list
of sgs which shares a page boundary into one and giving it to USB driver.
With this the number of sgs mapped it not equal to the the number of sgs
passed. Because of this USB driver is not marking the chain bit to false
since it couldn't iterate to the last sg. This patch addresses this issue
by marking the chain bit to false if it is the last mapped sg.

At a practical level, this patch resolves USB transfer stalls
seen with adb on dwc3 based db845c, pixel3 and other qcom
hardware after functionfs gadget added scatter-gather support
around v4.20.

Credit also to Anurag Kumar Vulisha <anurag.kumar.vulisha@xilinx.com>
who implemented a very similar fix to this issue.

Cc: Felipe Balbi <balbi@kernel.org>
Cc: Yang Fei <fei.yang@intel.com>
Cc: Thinh Nguyen <thinhn@synopsys.com>
Cc: Tejas Joglekar <tejas.joglekar@synopsys.com>
Cc: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Cc: Jack Pham <jackp@codeaurora.org>
Cc: Todd Kjos <tkjos@google.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Linux USB List <linux-usb@vger.kernel.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>
[jstultz: Slight tweak to remove sg_is_last() usage, reworked
          commit message, minor comment tweak]
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
 drivers/usb/dwc3/gadget.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
index 1b8014ab0b25..10aa511051e8 100644
--- a/drivers/usb/dwc3/gadget.c
+++ b/drivers/usb/dwc3/gadget.c
@@ -1071,7 +1071,14 @@ static void dwc3_prepare_one_trb_sg(struct dwc3_ep *dep,
 		unsigned int rem = length % maxp;
 		unsigned chain = true;
 
-		if (sg_is_last(s))
+		/*
+		 * IOMMU driver is coalescing the list of sgs which shares a
+		 * page boundary into one and giving it to USB driver. With
+		 * this the number of sgs mapped it not equal to the the number
+		 * of sgs passed. Mark the chain bit to false if it is the last
+		 * mapped sg.
+		 */
+		if ((i == remaining - 1))
 			chain = false;
 
 		if (rem && usb_endpoint_dir_out(dep->endpoint.desc) && !chain) {
-- 
2.17.1

Re: [PATCH] usb: dwc3: gadget: Update chain bit correctly when using sg list

[Jack Pham]

Hi John,

Thanks for following-up with this! While you're doing minor tweaks
anyway, I hope you don't mind me picking some nits below.

On Tue, Feb 18, 2020 at 11:51:04PM +0000, John Stultz wrote:
> From: Pratham Pratap <prathampratap@codeaurora.org>
> 
> If scatter-gather operation is allowed, a large USB request is split
> into multiple TRBs. For preparing TRBs for sg list, driver iterates
> over the list and creates TRB for each sg and mark the chain bit to
> false for the last sg. The current IOMMU driver is clubbing the list
> of sgs which shares a page boundary into one and giving it to USB driver.
> With this the number of sgs mapped it not equal to the the number of sgs
> passed. Because of this USB driver is not marking the chain bit to false
> since it couldn't iterate to the last sg. This patch addresses this issue
> by marking the chain bit to false if it is the last mapped sg.
> 
> At a practical level, this patch resolves USB transfer stalls
> seen with adb on dwc3 based db845c, pixel3 and other qcom
> hardware after functionfs gadget added scatter-gather support
> around v4.20.
> 
> Credit also to Anurag Kumar Vulisha <anurag.kumar.vulisha@xilinx.com>
> who implemented a very similar fix to this issue.
> 
> Cc: Felipe Balbi <balbi@kernel.org>
> Cc: Yang Fei <fei.yang@intel.com>
> Cc: Thinh Nguyen <thinhn@synopsys.com>
> Cc: Tejas Joglekar <tejas.joglekar@synopsys.com>
> Cc: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
> Cc: Jack Pham <jackp@codeaurora.org>
> Cc: Todd Kjos <tkjos@google.com>
> Cc: Greg KH <gregkh@linuxfoundation.org>
> Cc: Linux USB List <linux-usb@vger.kernel.org>
> Cc: stable <stable@vger.kernel.org>
> Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>
> [jstultz: Slight tweak to remove sg_is_last() usage, reworked
>           commit message, minor comment tweak]
> Signed-off-by: John Stultz <john.stultz@linaro.org>
> ---
>  drivers/usb/dwc3/gadget.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
> index 1b8014ab0b25..10aa511051e8 100644
> --- a/drivers/usb/dwc3/gadget.c
> +++ b/drivers/usb/dwc3/gadget.c
> @@ -1071,7 +1071,14 @@ static void dwc3_prepare_one_trb_sg(struct dwc3_ep *dep,
>  		unsigned int rem = length % maxp;
>  		unsigned chain = true;
>  
> -		if (sg_is_last(s))
> +		/*
> +		 * IOMMU driver is coalescing the list of sgs which shares a
> +		 * page boundary into one and giving it to USB driver. With
> +		 * this the number of sgs mapped it not equal to the the number
                                                 ^^ s/it/is/     ^^^ /d

Or could we more specifically say "number of sgs mapped could be less
than number passed"?

> +		 * of sgs passed. Mark the chain bit to false if it is the last
> +		 * mapped sg.
> +		 */
> +		if ((i == remaining - 1))

These outer parens are superfluous.

Also wondering if it would be more or less clear to just set the
variable once (and awkwardly move the comment to appear above the
local var declaration):

		unsigned chain = (i < remaining - 1);

>  			chain = false;
>  
>  		if (rem && usb_endpoint_dir_out(dep->endpoint.desc) && !chain) {

Jack
-- 
The Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

Re: [PATCH] usb: dwc3: gadget: Update chain bit correctly when using sg list

[John Stultz]

On Tue, Feb 18, 2020 at 4:07 PM Jack Pham <jackp@codeaurora.org> wrote:
>
> Hi John,
>
> Thanks for following-up with this! While you're doing minor tweaks
> anyway, I hope you don't mind me picking some nits below.
>
> On Tue, Feb 18, 2020 at 11:51:04PM +0000, John Stultz wrote:
> > From: Pratham Pratap <prathampratap@codeaurora.org>
> >
> > If scatter-gather operation is allowed, a large USB request is split
> > into multiple TRBs. For preparing TRBs for sg list, driver iterates
> > over the list and creates TRB for each sg and mark the chain bit to
> > false for the last sg. The current IOMMU driver is clubbing the list
> > of sgs which shares a page boundary into one and giving it to USB driver.
> > With this the number of sgs mapped it not equal to the the number of sgs
> > passed. Because of this USB driver is not marking the chain bit to false
> > since it couldn't iterate to the last sg. This patch addresses this issue
> > by marking the chain bit to false if it is the last mapped sg.
> >
> > At a practical level, this patch resolves USB transfer stalls
> > seen with adb on dwc3 based db845c, pixel3 and other qcom
> > hardware after functionfs gadget added scatter-gather support
> > around v4.20.
> >
> > Credit also to Anurag Kumar Vulisha <anurag.kumar.vulisha@xilinx.com>
> > who implemented a very similar fix to this issue.
> >
> > Cc: Felipe Balbi <balbi@kernel.org>
> > Cc: Yang Fei <fei.yang@intel.com>
> > Cc: Thinh Nguyen <thinhn@synopsys.com>
> > Cc: Tejas Joglekar <tejas.joglekar@synopsys.com>
> > Cc: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
> > Cc: Jack Pham <jackp@codeaurora.org>
> > Cc: Todd Kjos <tkjos@google.com>
> > Cc: Greg KH <gregkh@linuxfoundation.org>
> > Cc: Linux USB List <linux-usb@vger.kernel.org>
> > Cc: stable <stable@vger.kernel.org>
> > Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>
> > [jstultz: Slight tweak to remove sg_is_last() usage, reworked
> >           commit message, minor comment tweak]
> > Signed-off-by: John Stultz <john.stultz@linaro.org>
> > ---
> >  drivers/usb/dwc3/gadget.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
> > index 1b8014ab0b25..10aa511051e8 100644
> > --- a/drivers/usb/dwc3/gadget.c
> > +++ b/drivers/usb/dwc3/gadget.c
> > @@ -1071,7 +1071,14 @@ static void dwc3_prepare_one_trb_sg(struct dwc3_ep *dep,
> >               unsigned int rem = length % maxp;
> >               unsigned chain = true;
> >
> > -             if (sg_is_last(s))
> > +             /*
> > +              * IOMMU driver is coalescing the list of sgs which shares a
> > +              * page boundary into one and giving it to USB driver. With
> > +              * this the number of sgs mapped it not equal to the the number
>                                                  ^^ s/it/is/     ^^^ /d
>
> Or could we more specifically say "number of sgs mapped could be less
> than number passed"?
>
> > +              * of sgs passed. Mark the chain bit to false if it is the last
> > +              * mapped sg.
> > +              */
> > +             if ((i == remaining - 1))
>
> These outer parens are superfluous.

Thanks for catching these. I'll respin here shortly.

> Also wondering if it would be more or less clear to just set the
> variable once (and awkwardly move the comment to appear above the
> local var declaration):
>
>                 unsigned chain = (i < remaining - 1);
>

Personally, I think doing it via the conditional makes the logic a bit
less taxing to read/skim. So I might keep that bit as is.

thanks
-john

Re: [PATCH] usb: dwc3: gadget: Update chain bit correctly when using sg list

[Felipe Balbi]

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Hi,

John Stultz <john.stultz@linaro.org> writes:
>>                 unsigned chain = (i < remaining - 1);
>>
>
> Personally, I think doing it via the conditional makes the logic a bit
> less taxing to read/skim. So I might keep that bit as is.

I agree, it's easier to follow the code. Compiler is, mostly, likely
optimizing that anyway.

-- 
balbi

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