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Subject: Re: [PATCH] ubi: do not re-read the data already read out in retry
To: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
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Cc: linux-mtd@lists.infradead.org, Brian Norris <computersforpeace@gmail.com>
From: Richard Weinberger <richard@nod.at>
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Date: Fri, 21 Aug 2015 11:17:56 +0200
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Am 21.08.2015 um 09:59 schrieb Dongsheng Yang:
> In ubi_io_read(), we will retry if current reading failed
> to read all data we wanted. But we are doing a full re-do
> in the re-try path. Actually, we can skip the data which
> we have already read out in the last reading.
> 
> Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
> ---
>  drivers/mtd/ubi/io.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/mtd/ubi/io.c b/drivers/mtd/ubi/io.c
> index 5bbd1f0..a3ac643 100644
> --- a/drivers/mtd/ubi/io.c
> +++ b/drivers/mtd/ubi/io.c
> @@ -127,7 +127,7 @@ int ubi_io_read(const struct ubi_device *ubi, void *buf, int pnum, int offset,
>  		int len)
>  {
>  	int err, retries = 0;
> -	size_t read;
> +	size_t read, already_read = 0;
>  	loff_t addr;
>  
>  	dbg_io("read %d bytes from PEB %d:%d", len, pnum, offset);
> @@ -165,6 +165,7 @@ int ubi_io_read(const struct ubi_device *ubi, void *buf, int pnum, int offset,
>  	addr = (loff_t)pnum * ubi->peb_size + offset;
>  retry:
>  	err = mtd_read(ubi->mtd, addr, len, &read, buf);
> +	already_read += read;

Hmm, this change makes me nervous.

Brian, does MTD core guarantee that upon an erroneous mtd_read() the number of "read" bytes
in "buf" are valid?

So, my fear is that this change will introduce new regressions (due faulty MTD drivers, etc..)
without a real gain.

Thanks,
//richard