linux-kernel - Re: [PATCH] libata-sff: fix 32-bit PIO regression

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Date:	Sun, 8 Feb 2009 21:12:53 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Sergei Shtylyov <sshtylyov@...mvista.com>
cc:	jgarzik@...ox.com, linux-ide@...r.kernel.org,
	linux-kernel@...r.kernel.org, alan@...rguk.ukuu.org.uk, rjw@...k.pl
Subject: Re: [PATCH] libata-sff: fix 32-bit PIO regression

On Sun, 8 Feb 2009, Sergei Shtylyov wrote:

> Commit 871af1210f13966ab911ed2166e4ab2ce775b99d (libata: Add 32bit PIO support)
> caused all kind of errors on the ATAPI devices, so it's been empirically proven
> that one shouldn't read/write an extra data word when a device isn't expecting
> it already. "Don't do it then"; however still taking a chance to use 32-bit I/O
> one last  time when there are exactly 3 trailing bytes.  Oh, and stop pointless
> swapping bytes to and fro as well by using io*_rep() which shouldn't byte-swap.
> 
> This should fix the kernel.org bug #12609.
> 
> ---
> This is hopefully better replacement for Hugh Dickins most recent patch
> (http://marc.info/?l=linux-ide&m=123352294619281)...

Yes, looks nice, and works for me.  My only criticism would be,
minor issue unchanged by your patch, that actually "slop" isn't
unlikely enough to deserve an "unlikely" - slop of 1 or 3 is
unlikely, but slop of 2 is quite common.

Hugh

> 
>  drivers/ata/libata-sff.c |   23 ++++++++++++++++-------
>  1 files changed, 16 insertions(+), 7 deletions(-)
> 
> Index: linux-2.6/drivers/ata/libata-sff.c
> ===================================================================
> --- linux-2.6.orig/drivers/ata/libata-sff.c
> +++ linux-2.6/drivers/ata/libata-sff.c
> @@ -773,18 +773,27 @@ unsigned int ata_sff_data_xfer32(struct 
>  	else
>  		iowrite32_rep(data_addr, buf, words);
>  
> +	/* Transfer trailing bytes, if any */
>  	if (unlikely(slop)) {
> -		__le32 pad;
> +		unsigned char pad[4];
> +
> +		/* Point buf to the tail of buffer */
> +		buf += buflen - slop;
>  		if (rw == READ) {
> -			pad = cpu_to_le32(ioread32(ap->ioaddr.data_addr));
> -			memcpy(buf + buflen - slop, &pad, slop);
> +			if (slop < 3)
> +				ioread16_rep(data_addr, pad, 1);
> +			else
> +				ioread32_rep(data_addr, pad, 1);
> +			memcpy(buf, pad, slop);
>  		} else {
> -			memcpy(&pad, buf + buflen - slop, slop);
> -			iowrite32(le32_to_cpu(pad), ap->ioaddr.data_addr);
> +			memcpy(pad, buf, slop);
> +			if (slop < 3)
> +				iowrite16_rep(data_addr, pad, 1);
> +			else
> +				iowrite32_rep(data_addr, pad, 1);
>  		}
> -		words++;
>  	}
> -	return words << 2;
> +	return (buflen + 1) & ~1;
>  }
>  EXPORT_SYMBOL_GPL(ata_sff_data_xfer32);
>  
--
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