linux-kernel - Re: [PATCH v2 0/5] Add option to mmap GEM buffers cached

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Date:   Mon, 8 Mar 2021 09:41:19 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Paul Cercueil <paul@...pouillou.net>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     Sam Ravnborg <sam@...nborg.org>, od@...c.me,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-mips@...r.kernel.org
Subject: Re: [PATCH v2 0/5] Add option to mmap GEM buffers cached

Hi Paul,

having individual functions for each mode only makes sense if the 
decision is at compile time. But in patch 5, you're working around your 
earlier design by introducing in-driver helpers that select the correct 
CMA function.

In SHMEM helpers we have the flag map_wc in the GEM structure that 
selects the pages caching mode (wc vs uncached). I think CMA should use 
this design as well. Have a map_noncoherent flag in the CMA GEM object 
and set it from the driver's implementation of gem_create_object.

And in the long run, we could try to consolidate all drivers/helpers 
mapping flags in struct drm_gem_object.

Best regards
Thomas

Am 07.03.21 um 21:28 schrieb Paul Cercueil:
> Rework of my previous patchset which added support for GEM buffers
> backed by non-coherent memory to the ingenic-drm driver.
> 
> Having GEM buffers backed by non-coherent memory is interesting in
> the particular case where it is faster to render to a non-coherent
> buffer then sync the data cache, than to render to a write-combine
> buffer, and (by extension) much faster than using a shadow buffer.
> This is true for instance on some Ingenic SoCs, where even simple
> blits (e.g. memcpy) are about three times faster using this method.
> 
> For the record, the previous patchset was accepted for 5.10 then had
> to be reverted, as it conflicted with some changes made to the DMA API.
> 
> This new patchset is pretty different as it adds the functionality to
> the DRM core. The first three patches add variants to existing functions
> but with the "non-coherent memory" twist, exported as GPL symbols. The
> fourth patch adds a function to be used with the damage helpers.
> Finally, the last patch adds support for non-coherent GEM buffers to the
> ingenic-drm driver. The functionality is enabled through a module
> parameter, and is disabled by default.
> 
> Cheers,
> -Paul
> 
> Paul Cercueil (5):
>    drm: Add and export function drm_gem_cma_create_noncoherent
>    drm: Add and export function drm_gem_cma_dumb_create_noncoherent
>    drm: Add and export function drm_gem_cma_mmap_noncoherent
>    drm: Add and export function drm_gem_cma_sync_data
>    drm/ingenic: Add option to alloc cached GEM buffers
> 
>   drivers/gpu/drm/drm_gem_cma_helper.c      | 223 +++++++++++++++++++---
>   drivers/gpu/drm/ingenic/ingenic-drm-drv.c |  49 ++++-
>   drivers/gpu/drm/ingenic/ingenic-drm.h     |   4 +
>   drivers/gpu/drm/ingenic/ingenic-ipu.c     |  14 +-
>   include/drm/drm_gem_cma_helper.h          |  13 ++
>   5 files changed, 273 insertions(+), 30 deletions(-)
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer



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