[PATCH 1/1] dma-contiguous: return early for dt case in dma_contiguous_reserve

[PATCH 1/1] dma-contiguous: return early for dt case in dma_contiguous_reserve

[Dong Aisheng]

dma_contiguous_reserve() aims to support cmdline case for CMA memory
reserve. But if users define reserved memory in DT,
'dma_contiguous_default_area' will not be 0, then it's meaningless
to continue to run dma_contiguous_reserve(). So we return early
if detect 'dma_contiguous_default_area' is unzero.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
---
 kernel/dma/contiguous.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
index 3d63d91cba5c..ebade9f43eff 100644
--- a/kernel/dma/contiguous.c
+++ b/kernel/dma/contiguous.c
@@ -171,6 +171,9 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
 	phys_addr_t selected_limit = limit;
 	bool fixed = false;
 
+	if (dma_contiguous_default_area)
+		return;
+
 	pr_debug("%s(limit %08lx)\n", __func__, (unsigned long)limit);
 
 	if (size_cmdline != -1) {
@@ -191,7 +194,7 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
 #endif
 	}
 
-	if (selected_size && !dma_contiguous_default_area) {
+	if (selected_size) {
 		pr_debug("%s: reserving %ld MiB for global area\n", __func__,
 			 (unsigned long)selected_size / SZ_1M);
 
-- 
2.25.1

Re: [PATCH 1/1] dma-contiguous: return early for dt case in dma_contiguous_reserve

[Robin Murphy]

On 2021-05-18 12:28, Dong Aisheng wrote:
> dma_contiguous_reserve() aims to support cmdline case for CMA memory
> reserve. But if users define reserved memory in DT,
> 'dma_contiguous_default_area' will not be 0, then it's meaningless
> to continue to run dma_contiguous_reserve(). So we return early
> if detect 'dma_contiguous_default_area' is unzero.

But dma_contiguous_default_area *shouldn't* be set if the command-line 
argument is present - see the "if (size_cmdline != -1 && default_cma)" 
part of rmem_cma_setup(). Are you seeing something different in practice?

> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> Cc: Robin Murphy <robin.murphy@arm.com>
> Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
> ---
>   kernel/dma/contiguous.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
> index 3d63d91cba5c..ebade9f43eff 100644
> --- a/kernel/dma/contiguous.c
> +++ b/kernel/dma/contiguous.c
> @@ -171,6 +171,9 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
>   	phys_addr_t selected_limit = limit;
>   	bool fixed = false;
>   
> +	if (dma_contiguous_default_area)
> +		return;
> +
>   	pr_debug("%s(limit %08lx)\n", __func__, (unsigned long)limit);
>   
>   	if (size_cmdline != -1) {
> @@ -191,7 +194,7 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
>   #endif
>   	}
>   
> -	if (selected_size && !dma_contiguous_default_area) {
> +	if (selected_size) {

Either way, does skipping a handful of trivial calculations and a 
debugging message really matter even when it is redundant? I can't 
imagine it has any measurable effect on boot times...

Robin.

>   		pr_debug("%s: reserving %ld MiB for global area\n", __func__,
>   			 (unsigned long)selected_size / SZ_1M);
>   
> 

Re: [PATCH 1/1] dma-contiguous: return early for dt case in dma_contiguous_reserve

[Dong Aisheng]

On Wed, May 19, 2021 at 2:51 AM Robin Murphy <robin.murphy@arm.com> wrote:
>
> On 2021-05-18 12:28, Dong Aisheng wrote:
> > dma_contiguous_reserve() aims to support cmdline case for CMA memory
> > reserve. But if users define reserved memory in DT,
> > 'dma_contiguous_default_area' will not be 0, then it's meaningless
> > to continue to run dma_contiguous_reserve(). So we return early
> > if detect 'dma_contiguous_default_area' is unzero.
>
> But dma_contiguous_default_area *shouldn't* be set if the command-line
> argument is present - see the "if (size_cmdline != -1 && default_cma)"
> part of rmem_cma_setup(). Are you seeing something different in practice?
>

yes, you're right.

> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> > Cc: Robin Murphy <robin.murphy@arm.com>
> > Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
> > ---
> >   kernel/dma/contiguous.c | 5 ++++-
> >   1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
> > index 3d63d91cba5c..ebade9f43eff 100644
> > --- a/kernel/dma/contiguous.c
> > +++ b/kernel/dma/contiguous.c
> > @@ -171,6 +171,9 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
> >       phys_addr_t selected_limit = limit;
> >       bool fixed = false;
> >
> > +     if (dma_contiguous_default_area)
> > +             return;
> > +
> >       pr_debug("%s(limit %08lx)\n", __func__, (unsigned long)limit);
> >
> >       if (size_cmdline != -1) {
> > @@ -191,7 +194,7 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
> >   #endif
> >       }
> >
> > -     if (selected_size && !dma_contiguous_default_area) {
> > +     if (selected_size) {
>
> Either way, does skipping a handful of trivial calculations and a
> debugging message really matter even when it is redundant? I can't
> imagine it has any measurable effect on boot times...
>

I think it's not about performance.
It aims to improve the code readability as it's meaningless to continue to
execute cmdline CMA reserve logic once DT is used successfully which is
a bit confusing when people first read this part of code.
Does it make sense to you?

Regards
Aisheng

> Robin.
>
> >               pr_debug("%s: reserving %ld MiB for global area\n", __func__,
> >                        (unsigned long)selected_size / SZ_1M);
> >
> >

Re: [PATCH 1/1] dma-contiguous: return early for dt case in dma_contiguous_reserve

[Dong Aisheng]

On Tue, May 18, 2021 at 7:29 PM Dong Aisheng <aisheng.dong@nxp.com> wrote:
>
> dma_contiguous_reserve() aims to support cmdline case for CMA memory
> reserve. But if users define reserved memory in DT,
> 'dma_contiguous_default_area' will not be 0, then it's meaningless
> to continue to run dma_contiguous_reserve(). So we return early
> if detect 'dma_contiguous_default_area' is unzero.
>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> Cc: Robin Murphy <robin.murphy@arm.com>
> Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>

Gently ping

Regards
Aisheng

> ---
>  kernel/dma/contiguous.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
> index 3d63d91cba5c..ebade9f43eff 100644
> --- a/kernel/dma/contiguous.c
> +++ b/kernel/dma/contiguous.c
> @@ -171,6 +171,9 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
>         phys_addr_t selected_limit = limit;
>         bool fixed = false;
>
> +       if (dma_contiguous_default_area)
> +               return;
> +
>         pr_debug("%s(limit %08lx)\n", __func__, (unsigned long)limit);
>
>         if (size_cmdline != -1) {
> @@ -191,7 +194,7 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
>  #endif
>         }
>
> -       if (selected_size && !dma_contiguous_default_area) {
> +       if (selected_size) {
>                 pr_debug("%s: reserving %ld MiB for global area\n", __func__,
>                          (unsigned long)selected_size / SZ_1M);
>
> --
> 2.25.1
>

Re: [PATCH 1/1] dma-contiguous: return early for dt case in dma_contiguous_reserve

[Robin Murphy]

On 2021-05-31 10:21, Dong Aisheng wrote:
> On Tue, May 18, 2021 at 7:29 PM Dong Aisheng <aisheng.dong@nxp.com> wrote:
>>
>> dma_contiguous_reserve() aims to support cmdline case for CMA memory
>> reserve. But if users define reserved memory in DT,
>> 'dma_contiguous_default_area' will not be 0, then it's meaningless
>> to continue to run dma_contiguous_reserve(). So we return early
>> if detect 'dma_contiguous_default_area' is unzero.
>>
>> Cc: Christoph Hellwig <hch@lst.de>
>> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
>> Cc: Robin Murphy <robin.murphy@arm.com>
>> Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
> 
> Gently ping

The commit message is still wrong, and I still think the change doesn't 
achieve anything meaningful. This code is hard to make sense of either 
way because the crucial interplay between size_cmdline and 
dma_contiguous_default_area is hidden somewhere else entirely, and it 
would take a much more significant refactoring to clear that up.

Robin.

> 
> Regards
> Aisheng
> 
>> ---
>>   kernel/dma/contiguous.c | 5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
>> index 3d63d91cba5c..ebade9f43eff 100644
>> --- a/kernel/dma/contiguous.c
>> +++ b/kernel/dma/contiguous.c
>> @@ -171,6 +171,9 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
>>          phys_addr_t selected_limit = limit;
>>          bool fixed = false;
>>
>> +       if (dma_contiguous_default_area)
>> +               return;
>> +
>>          pr_debug("%s(limit %08lx)\n", __func__, (unsigned long)limit);
>>
>>          if (size_cmdline != -1) {
>> @@ -191,7 +194,7 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
>>   #endif
>>          }
>>
>> -       if (selected_size && !dma_contiguous_default_area) {
>> +       if (selected_size) {
>>                  pr_debug("%s: reserving %ld MiB for global area\n", __func__,
>>                           (unsigned long)selected_size / SZ_1M);
>>
>> --
>> 2.25.1
>>