Re: [PATCH 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

[Vivek Gautam <vivek.gautam@codeaurora.org>]

Hi Tomasz, Jordan,


On 11/21/2018 9:18 AM, Tomasz Figa wrote:
> Hi Jordan, Vivek,
>
> On Wed, Nov 21, 2018 at 12:41 AM Jordan Crouse <jcrouse@codeaurora.org> wrote:
>> On Tue, Nov 20, 2018 at 03:24:37PM +0530, Vivek Gautam wrote:
>>> dma_map_sg() expects a DMA domain. However, the drm devices
>>> have been traditionally using unmanaged iommu domain which
>>> is non-dma type. Using dma mapping APIs with that domain is bad.
>>>
>>> Replace dma_map_sg() calls with dma_sync_sg_for_device{|cpu}()
>>> to do the cache maintenance.
>>>
>>> Signed-off-by: Vivek Gautam <vivek.gautam@codeaurora.org>
>>> Suggested-by: Tomasz Figa <tfiga@chromium.org>
>>> ---
>>>
>>> Tested on an MTP sdm845:
>>> https://github.com/vivekgautam1/linux/tree/v4.19/sdm845-mtp-display-working
>>>
>>>   drivers/gpu/drm/msm/msm_gem.c | 27 ++++++++++++++++++++-------
>>>   1 file changed, 20 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
>>> index 00c795ced02c..d7a7af610803 100644
>>> --- a/drivers/gpu/drm/msm/msm_gem.c
>>> +++ b/drivers/gpu/drm/msm/msm_gem.c
>>> @@ -81,6 +81,8 @@ static struct page **get_pages(struct drm_gem_object *obj)
>>>                struct drm_device *dev = obj->dev;
>>>                struct page **p;
>>>                int npages = obj->size >> PAGE_SHIFT;
>>> +             struct scatterlist *s;
>>> +             int i;
>>>
>>>                if (use_pages(obj))
>>>                        p = drm_gem_get_pages(obj);
>>> @@ -107,9 +109,19 @@ static struct page **get_pages(struct drm_gem_object *obj)
>>>                /* For non-cached buffers, ensure the new pages are clean
>>>                 * because display controller, GPU, etc. are not coherent:
>>>                 */
>>> -             if (msm_obj->flags & (MSM_BO_WC|MSM_BO_UNCACHED))
>>> -                     dma_map_sg(dev->dev, msm_obj->sgt->sgl,
>>> -                                     msm_obj->sgt->nents, DMA_BIDIRECTIONAL);
>>> +             if (msm_obj->flags & (MSM_BO_WC | MSM_BO_UNCACHED)) {
>>> +                     /*
>>> +                      * Fake up the SG table so that dma_sync_sg_*()
>>> +                      * can be used to flush the pages associated with it.
>>> +                      */
>> We aren't really faking.  The table is real, we are just slightly abusing the
>> sg_dma_address() which makes this comment a bit misleading. Instead I would
>> probably say something like:
>>
>> /* dma_sync_sg_* flushes pages using sg_dma_address() so point it at the
>>   * physical page for the right behavior */
>>
>> Or something like that.
>>
> It's actually quite complicated, but I agree that the comment isn't
> very precise. The cases are as follows:
> - arm64 iommu_dma_ops use sg_phys()
> https://elixir.bootlin.com/linux/v4.20-rc3/source/arch/arm64/mm/dma-mapping.c#L599
> - swiotlb_dma_ops used on arm64 if no IOMMU is available use
> sg->dma_address directly:
> https://elixir.bootlin.com/linux/v4.20-rc3/source/kernel/dma/swiotlb.c#L832
> - arm_dma_ops use sg_dma_address():
> https://elixir.bootlin.com/linux/v4.20-rc3/source/arch/arm/mm/dma-mapping.c#L1130
> - arm iommu_ops use sg_page():
> https://elixir.bootlin.com/linux/v4.20-rc3/source/arch/arm/mm/dma-mapping.c#L1869
>
> Sounds like a mess...
Thanks for the review.

Technically with the below assignment we address all of the above. How 
about an even
simpler version of the suggested comment:

/* dma_sync_sg_* flushes physical pages, so point sg->dma_address to
  * the physical one for the right behavior.
  */


>
>>> +                     for_each_sg(msm_obj->sgt->sgl, s,
>>> +                                 msm_obj->sgt->nents, i)
>>> +                             sg_dma_address(s) = sg_phys(s);
>>> +
>> I'm wondering - wouldn't we want to do this association for cached buffers to so
>> we could sync them correctly in cpu_prep and cpu_fini?  Maybe it wouldn't hurt
>> to put this association in the main path (obviously the sync should stay inside
>> the conditional for uncached buffers).
>>

Sure, I will move this out of the conditional check.

> I guess it wouldn't hurt indeed. Note that cpu_prep/fini seem to be
> missing the sync call currently.

I can't say I understand the usage of cpu_prep and cpu_fini(). But I can add
the necessary support if you can point me in the right direction.
Thanks

Best regards
Vivek
>
> P.S. Jordan, not sure if it's my Gmail or your email client, but your
> message had all the recipients in a Reply-to header, except you, so
> pressing Reply to all in my case led to a message that didn't have you
> in recipients anymore...
>
> Best regards,
> Tomasz