Re: [Qemu-devel] [PATCH v2 3/3] block: get max_transfer limit for char (scsi-generic) devices

[Fam Zheng <famz@redhat.com>]

On Thu, 01/19 21:51, Eric Farman wrote:
> Commit 6f607174 introduced a routine to get the maximum number
> of bytes for a single I/O transfer for block devices, however
> scsi generic devices are character devices, not block.  Add
> a condition for this, such that scsi generic devices can view
> the same data.
> 
> Some tweaking of data is required, because the block and sg
> ioctls return values in different scales (sectors versus
> bytes).  So adjust hdev_get_max_transfer_length such that it
> always returns a value in bytes.
> 
> Signed-off-by: Eric Farman <farman@linux.vnet.ibm.com>
> ---
>  block/file-posix.c    | 10 ++++++----
>  include/block/block.h |  1 +
>  2 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index 2115155..94068ca 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -657,9 +657,11 @@ static int hdev_get_max_transfer_length(BlockDriverState *bs, int fd)
>      int max_sectors = 0;
>      short max_sectors_short = 0;
>      if (bs->sg && ioctl(fd, BLKSECTGET, &max_sectors) == 0) {
> +        /* sg returns a value in bytes */
>          return max_sectors;

The variable name is now misleading, maybe use "bytes" instead?

>      } else if (!bs->sg && ioctl(fd, BLKSECTGET, &max_sectors_short) == 0) {
> -        return max_sectors_short;
> +        /* block returns a value in sectors */
> +        return max_sectors_short << BDRV_SECTOR_BITS;
>      } else {
>          return -errno;
>      }
> @@ -674,10 +676,10 @@ static void raw_refresh_limits(BlockDriverState *bs, Error **errp)
>      struct stat st;
>  
>      if (!fstat(s->fd, &st)) {
> -        if (S_ISBLK(st.st_mode)) {
> +        if (S_ISBLK(st.st_mode) || S_ISCHR(st.st_mode)) {
>              int ret = hdev_get_max_transfer_length(bs, s->fd);
> -            if (ret > 0 && ret <= BDRV_REQUEST_MAX_SECTORS) {
> -                bs->bl.max_transfer = pow2floor(ret << BDRV_SECTOR_BITS);
> +            if (ret > 0 && ret <= BDRV_REQUEST_MAX_BYTES) {
> +                bs->bl.max_transfer = pow2floor(ret);
>              }
>          }
>      }
> diff --git a/include/block/block.h b/include/block/block.h
> index 8b0dcda..4e81f20 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -116,6 +116,7 @@ typedef struct HDGeometry {
>  
>  #define BDRV_REQUEST_MAX_SECTORS MIN(SIZE_MAX >> BDRV_SECTOR_BITS, \
>                                       INT_MAX >> BDRV_SECTOR_BITS)
> +#define BDRV_REQUEST_MAX_BYTES (BDRV_REQUEST_MAX_SECTORS << BDRV_SECTOR_BITS)
>  
>  /*
>   * Allocation status flags
> -- 
> 2.8.4
> 
>