[U-Boot] [PATCH 6/9] CACHE: nand read/write: Test if start address is aligned

[Scott Wood]

On 06/24/2012 07:17 PM, Marek Vasut wrote:
> This prevents the scenario where data cache is on and the
> device uses DMA to deploy data. In that case, it might not
> be possible to flush/invalidate data to RAM properly. The
> other option is to use bounce buffer,

Or get cache coherent hardware. :-)

> but that involves a lot of copying and therefore degrades performance
> rapidly. Therefore disallow this possibility of unaligned load
> address altogether if data cache is on.

How about use the bounce buffer only if the address is misaligned?  The
corrective action a user has to take is the same as with this patch,
except for an additional option of living with the slight performance
penalty.  How often does this actually happen?  How much does it
actually slow things down compared to the speed of the NAND chip?

I'm hesitant to break something -- even if it's odd (literally in this
case) -- that currently works on most hardware, just because one or two
drivers can't handle it.  It feels kind of like changing the read() and
write() system calls to require cacheline alignment. :-P

> Signed-off-by: Marek Vasut <marex at denx.de>
> Cc: Scott Wood <scottwood at freescale.com>
> ---
>  common/cmd_nand.c |    9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/common/cmd_nand.c b/common/cmd_nand.c
> index a91ccf4..122a91c 100644
> --- a/common/cmd_nand.c
> +++ b/common/cmd_nand.c
> @@ -609,6 +609,8 @@ int do_nand(cmd_tbl_t * cmdtp, int flag, int argc, char * const argv[])
>  			goto usage;
>  
>  		addr = (ulong)simple_strtoul(argv[2], NULL, 16);
> +		if (!cacheline_aligned(addr))
> +			return 1;

There's no way you can just return like this without printing an error
that lets the user know that the operation wasn't performed, and why.

-Scott