block: add a error_count field to struct request

[Bart Van Assche]

On Tue, 2017-04-18 at 08:52 -0700, Christoph Hellwig wrote:
> From: Christoph Hellwig <hch at lst.de>
> 
> This is for the legacy floppy and ataflop drivers that currently abuse
> ->errors for this purpose.  It's stashed away in a union to not grow
> the struct size, the other fields are either used by modern drivers
> for different purposes or the I/O scheduler before queing the I/O
> to drivers.
> 
> Signed-off-by: Christoph Hellwig <hch at lst.de>
> ---
>  include/linux/blkdev.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 5986e1250d7d..e618164462e8 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -175,6 +175,7 @@ struct request {
>  		struct rb_node rb_node;	/* sort/lookup */
>  		struct bio_vec special_vec;
>  		void *completion_data;
> +		int error_count; /* for legacy drivers, don't use */
>  	};
>  
>  	/*

Hello Christoph,

Both blk-mq and the traditional block layer support a .cmd_size field to
make the block layer core allocate driver-specific data at the end of struct
request. Could that mechanism have been used for the error_count field?

Thanks,

Bart.