Re: [PATCH v2 21/43] drm/fbdev-dma: Implement damage handling and deferred I/O

Thomas Zimmermann
2024-04-16T12:44:54Z

Hi

Am 16.04.24 um 14:18 schrieb Javier Martinez Canillas:
> Thomas Zimmermann <tzimmermann@suse.de> writes:
>
>> Add support for damage handling and deferred I/O to fbdev-dma. This
>> enables fbdev-dma to support all DMA-memory-based DRM drivers, even
>> such with a dirty callback in their framebuffers.
>>
>> The patch adds the code for deferred I/O and also sets a dedicated
>> helper for struct fb_ops.fb_mmap that support coherent mappings.
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
>> ---
>> drivers/gpu/drm/drm_fbdev_dma.c | 65 ++++++++++++++++++++++++++-------
>> 1 file changed, 51 insertions(+), 14 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_fbdev_dma.c b/drivers/gpu/drm/drm_fbdev_dma.c
>> index 6c9427bb4053b..8ffd072368bca 100644
>> --- a/drivers/gpu/drm/drm_fbdev_dma.c
>> +++ b/drivers/gpu/drm/drm_fbdev_dma.c
>> @@ -4,6 +4,7 @@
>>
>> #include <drm/drm_crtc_helper.h>
>> #include <drm/drm_drv.h>
>> +#include <drm/drm_fb_dma_helper.h>
>> #include <drm/drm_fb_helper.h>
>> #include <drm/drm_framebuffer.h>
>> #include <drm/drm_gem_dma_helper.h>
>> @@ -35,6 +36,22 @@ static int drm_fbdev_dma_fb_release(struct fb_info *info, int user)
>> return 0;
>> }
>>
>> +FB_GEN_DEFAULT_DEFERRED_SYSMEM_OPS(drm_fbdev_dma,
>> + drm_fb_helper_damage_range,
>> + drm_fb_helper_damage_area);
>> +
> Shouldn't this be FB_GEN_DEFAULT_DEFERRED_DMAMEM_OPS() instead ?
>
> I know that right now the macros are the same but I believe that it was
> added it for a reason ?

Oh, thanks for noticing! I asked for that macro specifically for this
reason. It went through the omap tree and hadn't arrived in
drm-misc-next when I first made these patches. I'll update the patch
accordingly.

>
>> +static int drm_fbdev_dma_fb_mmap(struct fb_info *info, struct vm_area_struct *vma)
>> +{
>> + struct drm_fb_helper *fb_helper = info->par;
>> + struct drm_framebuffer *fb = fb_helper->fb;
>> + struct drm_gem_dma_object *dma = drm_fb_dma_get_gem_obj(fb, 0);
>> +
>> + if (!dma->map_noncoherent)
>> + vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
> I noticed that some drivers do:
>
> vma->vm_page_prot = pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
>
> I see that vm_get_page_prot() is a per-architecture function, but I don't
> know about the implications of getting the pgprot_t from the vma->vm_flags
> set or just using the current vma->vm_page_prot value...

That's in interesting observation. The code in the patch adds a WC flag
to the existing vm_page_prot. The code in your example first creates a
new vm_page_prot from the vm_flags field. Fbdev drivers generally use
the former approach. So where does the original vm_page_prot value come
from? (I think that's also the question behind your comment.)

I've looked through the kernel's mmap code from the syscall [1] to the
place where it invokes the mmap callback. [2] Shortly before doing so,
mmap_region() set's vm_page_prot from vm_flags like in your example. [3]
I would assume there's no reason for drivers to call vm_get_page_prot()
by themselves. DRM drivers specially seem to have the habit of doing so.

Best regards
Thomas

[1]
https://elixir.bootlin.com/linux/v6.8/source/arch/x86/kernel/sys_x86_64.c#L86
[2] https://elixir.bootlin.com/linux/v6.8/source/mm/mmap.c#L2829
[3] https://elixir.bootlin.com/linux/v6.8/source/mm/mmap.c#L2824

>
> Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
>
> --
> Best regards,
>
> Javier Martinez Canillas
> Core Platforms
> Red Hat
>

--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)