On Fri, Nov 1, 2013 at 10:54 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, 30 Oct 2013, Ming Lei wrote:
>
>> We have sg_miter_* APIs for accessing scsi sg buffer, so
>> use them to make code clean and bug free.
>
> Hmmm. You could simply call sg_copy_buffer, if you didn't mind the
> quadratic penalty for the sg_miter_skip operations.
I don't want to change all current callers now.
>
>> --- a/drivers/usb/storage/protocol.c
>> +++ b/drivers/usb/storage/protocol.c
>> @@ -135,69 +135,43 @@ unsigned int usb_stor_access_xfer_buf(unsigned char *buffer,
>> unsigned int buflen, struct scsi_cmnd *srb, struct scatterlist **sgptr,
>> unsigned int *offset, enum xfer_buf_dir dir)
>> {
>> - unsigned int cnt;
>> + unsigned int cnt = 0;
>> struct scatterlist *sg = *sgptr;
>> + struct sg_mapping_iter miter;
>> + unsigned int nents = scsi_sg_count(srb);
>>
>> - /* We have to go through the list one entry
>> - * at a time. Each s-g entry contains some number of pages, and
>> - * each page has to be kmap()'ed separately. If the page is already
>> - * in kernel-addressable memory then kmap() will return its address.
>> - * If the page is not directly accessible -- such as a user buffer
>> - * located in high memory -- then kmap() will map it to a temporary
>> - * position in the kernel's virtual address space.
>> - */
>> -
>> - if (!sg)
>> + if (sg)
>> + nents -= sg - scsi_sglist(srb);
>
> This is definitely wrong. Scatterlist entries are not always stored in
> a linear array. To do this properly, you would have to make the caller
> keep track of the current value of nents.
>
> Or better yet, have the caller store and pass the sg_mapping_iter
> structure rather than sgptr and offset.
You are right, but looks we can use sg_nents(), which is easier, :-)
>
>> + else
>> sg = scsi_sglist(srb);
>>
>> - /* This loop handles a single s-g list entry, which may
>> - * include multiple pages. Find the initial page structure
>> - * and the starting offset within the page, and update
>> - * the *offset and **sgptr values for the next loop.
>> - */
>> - cnt = 0;
>> - while (cnt < buflen && sg) {
>> - struct page *page = sg_page(sg) +
>> - ((sg->offset + *offset) >> PAGE_SHIFT);
>> - unsigned int poff = (sg->offset + *offset) & (PAGE_SIZE-1);
>> - unsigned int sglen = sg->length - *offset;
>> -
>> - if (sglen > buflen - cnt) {
>> -
>> - /* Transfer ends within this s-g entry */
>> - sglen = buflen - cnt;
>> - *offset += sglen;
>> - } else {
>> + if (dir == FROM_XFER_BUF)
>> + sg_miter_start(&miter, sg, nents, SG_MITER_FROM_SG);
>> + else
>> + sg_miter_start(&miter, sg, nents, SG_MITER_TO_SG);
>
> I find this style somewhat annoying. Maybe the compiler is smart
> enough to optimize it, maybe not. In any case, I would prefer to see
>
> if (dir == FROM_XFER_BUF)
> sgdir = SG_MITER_FROM_SG;
> else
> sgdir = SG_MITER_TO_SG;
> sg_miter_start(&miter, nents, sgdir);
>
> Or even:
>
> sg_miter_start(&miter, nents, (dir == FROM_XFER_BUF ?
> SG_MITER_FROM_SG : SG_MITER_TO_SG));
Looks the above is fine.
>
>> - /* Transfer continues to next s-g entry */
>> - *offset = 0;
>> - sg = sg_next(sg);
>> - }
>> + if (!sg_miter_skip(&miter, *offset))
>> + return cnt;
>> +
>> + while (sg_miter_next(&miter) && cnt < buflen) {
>> + unsigned int len = min(miter.length, buflen - cnt);
>> +
>> + if (dir == FROM_XFER_BUF)
>> + memcpy(buffer + cnt, miter.addr, len);
>> + else
>> + memcpy(miter.addr, buffer + cnt, len);
>>
>> - /* Transfer the data for all the pages in this
>> - * s-g entry. For each page: call kmap(), do the
>> - * transfer, and call kunmap() immediately after. */
>> - while (sglen > 0) {
>> - unsigned int plen = min(sglen, (unsigned int)
>> - PAGE_SIZE - poff);
>> - unsigned char *ptr = kmap(page);
>> -
>> - if (dir == TO_XFER_BUF)
>> - memcpy(ptr + poff, buffer + cnt, plen);
>> - else
>> - memcpy(buffer + cnt, ptr + poff, plen);
>> - kunmap(page);
>> -
>> - /* Start at the beginning of the next page */
>> - poff = 0;
>> - ++page;
>> - cnt += plen;
>> - sglen -= plen;
>> + if (*offset + len < miter.piter.sg->length) {
>> + *offset += len;
>> + *sgptr = miter.piter.sg;
>> + } else {
>> + *offset = 0;
>> + *sgptr = sg_next(miter.piter.sg);
>> }
>> + cnt += len;
>> }
>> - *sgptr = sg;
>> + sg_miter_stop(&miter);
>>
>> - /* Return the amount actually transferred */
>
> Why remove this comment?
With this patch, the comment becomes uncessary since the code
is very simple and it is obvious.
Thanks,
--
Ming Lei
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