linux-kernel - Re: [PATCH V10 04/22] LoongArch: Add writecombine support for drm

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Date:   Sun, 15 May 2022 12:18:59 +0800
From:   WANG Xuerui <kernel@...0n.name>
To:     Huacai Chen <chenhuacai@...ngson.cn>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Airlie <airlied@...ux.ie>,
        Jonathan Corbet <corbet@....net>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Xuefeng Li <lixuefeng@...ngson.cn>,
        Yanteng Si <siyanteng@...ngson.cn>,
        Huacai Chen <chenhuacai@...il.com>,
        Guo Ren <guoren@...nel.org>, Xuerui Wang <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH V10 04/22] LoongArch: Add writecombine support for drm


On 5/14/22 16:03, Huacai Chen wrote:
> LoongArch maintains cache coherency in hardware, but its WUC attribute
> (Weak-ordered UnCached, which is similar to WC) is out of the scope of
> cache coherency machanism. This means WUC can only used for write-only
> memory regions.
>
> Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
> ---
>   drivers/gpu/drm/drm_vm.c         | 2 +-
>   drivers/gpu/drm/ttm/ttm_module.c | 2 +-
>   include/drm/drm_cache.h          | 8 ++++++++
>   3 files changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_vm.c b/drivers/gpu/drm/drm_vm.c
> index e957d4851dc0..f024dc93939e 100644
> --- a/drivers/gpu/drm/drm_vm.c
> +++ b/drivers/gpu/drm/drm_vm.c
> @@ -69,7 +69,7 @@ static pgprot_t drm_io_prot(struct drm_local_map *map,
>   	pgprot_t tmp = vm_get_page_prot(vma->vm_flags);
>   
>   #if defined(__i386__) || defined(__x86_64__) || defined(__powerpc__) || \
> -    defined(__mips__)
> +    defined(__mips__) || defined(__loongarch__)
>   	if (map->type == _DRM_REGISTERS && !(map->flags & _DRM_WRITE_COMBINING))
>   		tmp = pgprot_noncached(tmp);
>   	else
> diff --git a/drivers/gpu/drm/ttm/ttm_module.c b/drivers/gpu/drm/ttm/ttm_module.c
> index a3ad7c9736ec..b3fffe7b5062 100644
> --- a/drivers/gpu/drm/ttm/ttm_module.c
> +++ b/drivers/gpu/drm/ttm/ttm_module.c
> @@ -74,7 +74,7 @@ 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(__powerpc__) || defined(__mips__) || defined(__loongarch__)
>   	if (caching == ttm_write_combined)
>   		tmp = pgprot_writecombine(tmp);
>   	else
> diff --git a/include/drm/drm_cache.h b/include/drm/drm_cache.h
> index 22deb216b59c..08e0e3ffad13 100644
> --- a/include/drm/drm_cache.h
> +++ b/include/drm/drm_cache.h
> @@ -67,6 +67,14 @@ static inline bool drm_arch_can_wc_memory(void)
>   	 * optimization entirely for ARM and arm64.
>   	 */
>   	return false;
> +#elif defined(CONFIG_LOONGARCH)
> +	/*
> +	 * LoongArch maintains cache coherency in hardware, but its WUC attribute
> +	 * (Weak-ordered UnCached, which is similar to WC) is out of the scope of
> +	 * cache coherency machanism. This means WUC can only used for write-only
> +	 * memory regions.
> +	 */
> +	return false;
>   #else
>   	return true;
>   #endif

The code changes look reasonable, given the adequate comments, but have 
the drm people given acks? This seems to exclusively touch drm bits and 
not directly related to arch bring-up. (You may get scrambled screen 
output but everything else is working, I'm running my LoongArch devbox 
headlessly ever since I first set it up last year.)

If anything, IMO you could even take this patch out and still get the 
arch properly brought up. What do others think?

Nevertheless,

Reviewed-by: WANG Xuerui <git@...0n.name>