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SubjectRE: [PATCH] scsi: resolve COMMAND_SIZE at compile time
Date
From: Stephen Kitt
> Sent: 09 March 2018 22:34
>
> COMMAND_SIZE currently uses an array of values in block/scsi_ioctl.c.
> A number of device_handler functions use this to initialise arrays,
> and this is flagged by -Wvla.
>
> This patch replaces COMMAND_SIZE with a variant using a formula which
> can be resolved at compile time in cases where the opcode is fixed,
> resolving the array size and avoiding the VLA. The downside is that
> the code is less maintainable and that some call sites end up having
> more complex generated code.
>
> Since scsi_command_size_tbl is dropped, we can remove the dependency
> on BLK_SCSI_REQUEST from drivers/target/Kconfig.
>
> This was prompted by https://lkml.org/lkml/2018/3/7/621
>
> Signed-off-by: Stephen Kitt <steve@sk2.org>
> ---
> block/scsi_ioctl.c | 8 --------
> drivers/target/Kconfig | 1 -
> include/scsi/scsi_common.h | 13 +++++++++++--
> 3 files changed, 11 insertions(+), 11 deletions(-)
>
> diff --git a/block/scsi_ioctl.c b/block/scsi_ioctl.c
> index 60b471f8621b..b9e176e537be 100644
> --- a/block/scsi_ioctl.c
> +++ b/block/scsi_ioctl.c
> @@ -41,14 +41,6 @@ struct blk_cmd_filter {
>
> static struct blk_cmd_filter blk_default_cmd_filter;
>
> -/* Command group 3 is reserved and should never be used. */
> -const unsigned char scsi_command_size_tbl[8] =
> -{
> - 6, 10, 10, 12,
> - 16, 12, 10, 10
> -};
> -EXPORT_SYMBOL(scsi_command_size_tbl);
> -
> #include <scsi/sg.h>
>
> static int sg_get_version(int __user *p)
> diff --git a/drivers/target/Kconfig b/drivers/target/Kconfig
> index 4c44d7bed01a..b5f05b60cf06 100644
> --- a/drivers/target/Kconfig
> +++ b/drivers/target/Kconfig
> @@ -4,7 +4,6 @@ menuconfig TARGET_CORE
> depends on SCSI && BLOCK
> select CONFIGFS_FS
> select CRC_T10DIF
> - select BLK_SCSI_REQUEST # only for scsi_command_size_tbl..
> select SGL_ALLOC
> default n
> help
> diff --git a/include/scsi/scsi_common.h b/include/scsi/scsi_common.h
> index 731ac09ed231..48d950666376 100644
> --- a/include/scsi/scsi_common.h
> +++ b/include/scsi/scsi_common.h
> @@ -15,8 +15,17 @@ scsi_varlen_cdb_length(const void *hdr)
> return ((struct scsi_varlen_cdb_hdr *)hdr)->additional_cdb_length + 8;
> }
>
> -extern const unsigned char scsi_command_size_tbl[8];
> -#define COMMAND_SIZE(opcode) scsi_command_size_tbl[((opcode) >> 5) & 7]
> +/*
> + * SCSI command sizes are as follows, in bytes, for fixed size commands, per
> + * group: 6, 10, 10, 12, 16, 12, 10, 10. The top three bits of an opcode
> + * determine its group.
> + * The size table is encoded into a 32-bit value by subtracting each value
> + * from 16, resulting in a value of 1715488362
> + * (6 << 28 + 6 << 24 + 4 << 20 + 0 << 16 + 4 << 12 + 6 << 8 + 6 << 4 + 10).
> + * Command group 3 is reserved and should never be used.
> + */
> +#define COMMAND_SIZE(opcode) \
> + (16 - (15 & (1715488362 >> (4 * (((opcode) >> 5) & 7)))))

Why not:
(6 + (15 & (0x446a6440u ....

Specifying the constant in hex makes it more obvious.
It is a shame about the 16, but an offset is easier than the negate.

David

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