[RFC PATCH 0/2] drm: add a noncoherent mapping checking helper

[Conor Dooley]

On Wed, Sep 30, 2026 at 03:35:26PM +0800, Icenowy Zheng wrote:
> This patchset tries to address the lack of uncached mapping support on
> several SiFive-core-based SoCs, currently targeting JH7110 but EIC7700
> would have the same problem (although it uses DC8000 display controller
> instead of DC8200 and real display support is pending).
> 
> Because other users (either SoCs with Xuantie cores, which have
> Xtheadmae extensions or non-RISC-V SoCs) of the driver have proper
> writecombine support, a helper is added to check whether coherent
> (cache-bypassing) mapping is available. This helper currently returns
> true for all architectures except RISC-V, and checks whether
> pgprot_writecombine() is a no-op on RISC-V.
> 
> The patch really enabling manual cache maintenance for the verisilicon
> driver is picked from Dominique, and reworked to check both
> dma-noncoherent property and the platform's mapping capaibility.
> 
> Tested on VisionFive 2 with the display enablement patchset at [1] (and
> its dependency).
> 
> Also tested on Lichee Pi 4A (TH1520, which has Xtheadmae), operation is
> not affected, map_noncoherent isn't set and Xorg says kernel dirty
> updates aren't required.
> 
> [1] https://patch.msgid.link/20260929-jh7110-clean-send-v5-0-82b4d8e3c6c7@samsung.com

Out of curiosity, have you seen Bo Gan's series from a few months ago
that creates an extension/erratum to get these devices working?
https://lore.kernel.org/linux-riscv/20260316060328.1173634-1-ganboing@gmail.com/

I've been meaning to reply to it over and over again for the last number
of months, but I just haven't had the time to actually understand the
detail of the EIC7700 solution or check whether it works yet. I know
this is just one driver because the JH7110 only has this problem for the 
display pipeline, but according to Bo Gan this applies to all
peripherals on the EIC7700. 

> 
> Dominique Belhachemi (1):
>   drm: verisilicon: support non-coherent DMA framebuffers
> 
> Icenowy Zheng (1):
>   drm: add a helper function to check availbility of coherent mapping
> 
>  drivers/gpu/drm/verisilicon/vs_cursor_plane.c |  9 ++++++
>  drivers/gpu/drm/verisilicon/vs_drm.c          | 32 ++++++++++++++++++-
>  drivers/gpu/drm/verisilicon/vs_drm.h          |  7 ++++
>  .../gpu/drm/verisilicon/vs_primary_plane.c    |  9 ++++++
>  include/drm/drm_cache.h                       | 21 ++++++++++++
>  5 files changed, 77 insertions(+), 1 deletion(-)
> 
> -- 
> 2.55.0
> 
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