Re: [PATCH] mtd: spear_smi: Fix nonalignment not handled in memcpy_toio

[Boris Brezillon <boris.brezillon@collabora.com>]

On Fri, 18 Oct 2019 16:36:43 +0200
Miquel Raynal <miquel.raynal@bootlin.com> wrote:

> Any write with either dd or flashcp to a device driven by the
> spear_smi.c driver will pass through the spear_smi_cpy_toio()
> function. This function will get called for chunks of up to 256 bytes.
> If the amount of data is smaller, we may have a problem if the data
> length is not 4-byte aligned. In this situation, the kernel panics
> during the memcpy:
> 
>     # dd if=/dev/urandom bs=1001 count=1 of=/dev/mtd6
>     spear_smi_cpy_toio [620] dest c9070000, src c7be8800, len 256
>     spear_smi_cpy_toio [620] dest c9070100, src c7be8900, len 256
>     spear_smi_cpy_toio [620] dest c9070200, src c7be8a00, len 256
>     spear_smi_cpy_toio [620] dest c9070300, src c7be8b00, len 233
>     Unhandled fault: external abort on non-linefetch (0x808) at 0xc90703e8
>     [...]
>     PC is at memcpy+0xcc/0x330

Can you find out which instruction is at memcpy+0xcc/0x330? For the
record, the assembly is here [1].

> 
> Workaround this issue by using the alternate _memcpy_toio() method
> which at least does not present the same problem.
> 
> Fixes: f18dbbb1bfe0 ("mtd: ST SPEAr: Add SMI driver for serial NOR flash")
> Cc: stable@vger.kernel.org
> Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>

I don't remember suggesting that as a final solution. I probably
suggested to test with _memcpy_toio() to see if using a byte accessor
was fixing the problem, but it's definitely not the right solution
(using byte access with a memory barrier for 256 bytes buffers is likely
to cause a huge perf penalty).

> Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
> ---
> 
> Hello,
> 
> This patch could not be tested with a mainline kernel (only compiled)
> but was tested with a stable 4.14.x kernel. I have really no idea why
> memcpy fails in this situation that's why I propose this workaround
> but I bet there is something deeper not working.
> 
> Thanks,
> Miquèl
> 
>  drivers/mtd/devices/spear_smi.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/mtd/devices/spear_smi.c b/drivers/mtd/devices/spear_smi.c
> index 986f81d2f93e..d888625a3244 100644
> --- a/drivers/mtd/devices/spear_smi.c
> +++ b/drivers/mtd/devices/spear_smi.c
> @@ -614,7 +614,7 @@ static inline int spear_smi_cpy_toio(struct spear_smi *dev, u32 bank,
>  	ctrlreg1 = readl(dev->io_base + SMI_CR1);
>  	writel((ctrlreg1 | WB_MODE) & ~SW_MODE, dev->io_base + SMI_CR1);
>  
> -	memcpy_toio(dest, src, len);
> +	_memcpy_toio(dest, src, len);
>  
>  	writel(ctrlreg1, dev->io_base + SMI_CR1);
>  

[1]https://elixir.bootlin.com/linux/v5.4-rc2/source/arch/arm/lib/memcpy.S

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