linux-kernel - Re: [PATCH v2] drm/ttm: add pgprot handling for RISC-V

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Message-ID: <5fed2297-4e51-6aa9-464a-6f8a2cf8fcc6@kernel.org>
Date: Mon, 27 Oct 2025 10:41:01 -0600 (MDT)
From: Paul Walmsley <pjw@...nel.org>
To: Christian König <christian.koenig@....com>
cc: Icenowy Zheng <uwu@...nowy.me>, Huang Rui <ray.huang@....com>, 
    Matthew Auld <matthew.auld@...el.com>, 
    Matthew Brost <matthew.brost@...el.com>, 
    Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
    Maxime Ripard <mripard@...nel.org>, 
    Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
    Simona Vetter <simona@...ll.ch>, Paul Walmsley <paul.walmsley@...ive.com>, 
    Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
    Alexandre Ghiti <alex@...ti.fr>, dri-devel@...ts.freedesktop.org, 
    linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org, 
    Han Gao <rabenda.cn@...il.com>, Vivian Wang <wangruikang@...as.ac.cn>, 
    Inochi Amaoto <inochiama@...il.com>, Yao Zi <ziyao@...root.org>
Subject: Re: [PATCH v2] drm/ttm: add pgprot handling for RISC-V

On Mon, 27 Oct 2025, Christian König wrote:

> On 10/20/25 07:35, Icenowy Zheng wrote:
> > The RISC-V Svpbmt privileged extension provides support for overriding
> > page memory coherency attributes, and, along with vendor extensions like
> > Xtheadmae, supports pgprot_{writecombine,noncached} on RISC-V.
> > 
> > Adapt the codepath that maps ttm_write_combined to pgprot_writecombine
> > and ttm_noncached to pgprot_noncached to RISC-V, to allow proper page
> > access attributes.

[ ... ]

> > diff --git a/drivers/gpu/drm/ttm/ttm_module.c b/drivers/gpu/drm/ttm/ttm_module.c
> > index b3fffe7b5062a..aa137ead5cc59 100644
> > --- a/drivers/gpu/drm/ttm/ttm_module.c
> > +++ b/drivers/gpu/drm/ttm/ttm_module.c
> > @@ -74,7 +74,8 @@ pgprot_t ttm_prot_from_caching(enum ttm_caching caching, pgprot_t tmp)
> >  #endif /* CONFIG_UML */
> >  #endif /* __i386__ || __x86_64__ */
> >  #if defined(__ia64__) || defined(__arm__) || defined(__aarch64__) || \
> > -	defined(__powerpc__) || defined(__mips__) || defined(__loongarch__)
> > +	defined(__powerpc__) || defined(__mips__) || defined(__loongarch__) || \
> > +	defined(__riscv)
> 
> Looks reasonable, but does that work on all RISC-V variants?

>From an RISC-V architectural perspective, yes.

Of course there might be a hardware bug in some given manufacturer's 
implementation, but then again, that could happen on the other 
architectures as well.  


- Paul