linux-kernel - Re: [PATCH 1/2] memstick: add support for legacy memorysticks

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Date:	Thu, 9 Dec 2010 13:09:36 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Maxim Levitsky <maximlevitsky@...il.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Alex Dubov <oakad@...oo.com>, Takashi Iwai <tiwai@...e.de>
Subject: Re: [PATCH 1/2] memstick: add support for legacy memorysticks

On Thu,  9 Dec 2010 04:42:25 +0200
Maxim Levitsky <maximlevitsky@...il.com> wrote:

> +/*
> + * Advance scatterlist by 'consumed' bytes
> + * Returns new scatterlist, or NULL if can't advance that much
> + */
> +static struct scatterlist *sg_advance(struct scatterlist *sg, int consumed)
> +{
> +	while (consumed >= sg->length) {
> +		consumed -= sg->length;
> +
> +		sg = sg_next(sg);
> +		if (!sg)
> +			break;
> +	}
> +
> +	WARN_ON(!sg && consumed);
> +
> +	if (!sg)
> +		return NULL;
> +
> +	sg->offset += consumed;
> +	sg->length -= consumed;
> +
> +	if (sg->offset >= PAGE_SIZE) {
> +		struct page *page =
> +			nth_page(sg_page(sg), sg->offset / PAGE_SIZE);
> +		sg_set_page(sg, page, sg->length, sg->offset % PAGE_SIZE);
> +	}
> +
> +	return sg;
> +}
> +
> +/* Calculate number of sg entries in sg list */
> +static int sg_nents(struct scatterlist *sg)
> +{
> +	int nents = 0;
> +	while (sg) {
> +		nents++;
> +		sg = sg_next(sg);
> +	}
> +
> +	return nents;
> +}
> +
> +/* Calculate total lenght of scatterlist */

(typo)

> +static int sg_total_len(struct scatterlist *sg)
> +{
> +	int len = 0;
> +	while (sg) {
> +		len += sg->length;
> +		sg = sg_next(sg);
> +	}
> +	return len;
> +}
> +
> +/* Compare contents of an sg to a buffer */
> +static bool sg_compare_to_buffer(struct scatterlist *sg, u8 *buffer, size_t len)
> +{
> +	unsigned long flags;
> +	int retval = 0;
> +	struct sg_mapping_iter miter;
> +
> +	if (sg_total_len(sg) < len)
> +		return 1;
> +
> +	local_irq_save(flags);
> +	sg_miter_start(&miter, sg, sg_nents(sg),
> +				SG_MITER_ATOMIC | SG_MITER_FROM_SG);
> +
> +	while (sg_miter_next(&miter) && len > 0) {
> +
> +		int cmplen = min(miter.length, len);
> +		if (memcmp(miter.addr, buffer, cmplen)) {
> +			retval = 1;
> +			break;
> +		}
> +
> +		buffer += cmplen;
> +		len -= cmplen;
> +	}
> +
> +	sg_miter_stop(&miter);
> +	local_irq_restore(flags);
> +	return retval;
> +}

I think I mentioned this before, but ...  it's really bad that the
above functions appear in a particular device driver.  They are, to
varying degrees, independent of that driver and should be provided by
the sglist core code.

Also, the local_irq_save/restore in sg_compare_to_buffer() is quite
mysterious and should have a comment explaining why it is there.  Once
that's done, I might understand why it isn't a bug on SMP machines ;)

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