linux-kernel - Re: [RFC PATCH 1/3] pstore-ram: use write-combine mappings

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Date:	Tue, 9 Apr 2013 20:53:18 -0700
From:	Colin Cross <ccross@...roid.com>
To:	Rob Herring <robherring2@...il.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Rob Herring <rob.herring@...xeda.com>,
	Anton Vorontsov <cbouatmailru@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Tony Luck <tony.luck@...el.com>
Subject: Re: [RFC PATCH 1/3] pstore-ram: use write-combine mappings

On Tue, Apr 9, 2013 at 8:08 PM, Rob Herring <robherring2@...il.com> wrote:
> From: Rob Herring <rob.herring@...xeda.com>
>
> Atomic operations are undefined behavior on ARM for device or strongly
> ordered memory types. So use write-combine variants for mappings. This
> corresponds to normal, non-cacheable memory on ARM. For many other
> architectures, this change should not change the mapping type.

This is going to make ramconsole less reliable.  A debugging printk
followed by a __raw_writel that causes an immediate hard crash is
likely to lose the last updates, including the most useful message, in
the write buffers.

Also, isn't this patch unnecessary after patch 3 in this set?

> Signed-off-by: Rob Herring <rob.herring@...xeda.com>
> Cc: Anton Vorontsov <cbouatmailru@...il.com>
> Cc: Colin Cross <ccross@...roid.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Tony Luck <tony.luck@...el.com>
> Cc: linux-kernel@...r.kernel.org
> ---
>  fs/pstore/ram_core.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/pstore/ram_core.c b/fs/pstore/ram_core.c
> index 0306303..e126d9f 100644
> --- a/fs/pstore/ram_core.c
> +++ b/fs/pstore/ram_core.c
> @@ -337,7 +337,7 @@ static void *persistent_ram_vmap(phys_addr_t start, size_t size)
>         page_start = start - offset_in_page(start);
>         page_count = DIV_ROUND_UP(size + offset_in_page(start), PAGE_SIZE);
>
> -       prot = pgprot_noncached(PAGE_KERNEL);
> +       prot = pgprot_writecombine(PAGE_KERNEL);
Is this necessary?  Won't pgprot_noncached already be normal memory?

>         pages = kmalloc(sizeof(struct page *) * page_count, GFP_KERNEL);
>         if (!pages) {
> @@ -364,7 +364,7 @@ static void *persistent_ram_iomap(phys_addr_t start, size_t size)
>                 return NULL;
>         }
>
> -       return ioremap(start, size);
> +       return ioremap_wc(start, size);

ioremap_wc corresponds to MT_DEVICE_WC, which is still device memory,
so I don't see how this helps solve the problem in the commit message.

>  }
>
>  static int persistent_ram_buffer_map(phys_addr_t start, phys_addr_t size,
> --
> 1.7.10.4
>
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