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SubjectRe: [Patch] SCSI: Retrieve Cache Mode Using SG_ATA_16 if normal routine fails
On Tue, 11 Oct 2011, Amit Sahrawat wrote:

> SCSI: Retrieve Cache Mode Using SG_ATA_16 if normal routine fails
>
> It has been observed that a number of USB HDD's do not respond correctly
> to SCSI mode sense command(retrieve caching pages) which results in their
> Write Cache being discarded by queue requests i.e., WCE if left set to
> '0'(disabled). So, in order to identify the devices correctly - give it
> a last try using SG_ATA_16 after failure from normal routine.
>
> Signed-off-by: Amit Sahrawat <amit.sahrawat83@gmail.com>
>
> diff -Nurp linux-Orig/drivers/scsi/sd.c linux-Updated/drivers/scsi/sd.c
> --- linux-Orig/drivers/scsi/sd.c 2011-10-11 11:02:48.000000000 +0530
> +++ linux-Updated/drivers/scsi/sd.c 2011-10-11 11:10:09.000000000 +0530
> @@ -56,6 +56,7 @@
> #include <asm/uaccess.h>
> #include <asm/unaligned.h>
>
> +#include <scsi/sg.h>
> #include <scsi/scsi.h>
> #include <scsi/scsi_cmnd.h>
> #include <scsi/scsi_dbg.h>
> @@ -134,6 +135,58 @@ static const char *sd_cache_types[] = {
> "write back, no read (daft)"
> };
>
> +/* Relevant Structure and Function to be used to Retrieve
> + * Caching Information from USB HDD - this is picked from
> + * source code of 'hdparm'
> + *
> + *
> + * Definitions and structures for use with SG_IO + ATA_16:
> + * */
> +#define SG_ATA_16 0x85
> +#define SG_ATA_16_LEN 16
> +
> +#define ATA_OP_IDENTIFY 0xec
> +
> +/*
> + * Some useful ATA register bits
> + */
> +enum {
> + ATA_USING_LBA = (1 << 6),
> + ATA_STAT_DRQ = (1 << 3),
> + ATA_STAT_ERR = (1 << 0),
> +};
> +
> +struct ata_lba_regs {
> + __u8 feat;
> + __u8 nsect;
> + __u8 lbal;
> + __u8 lbam;
> + __u8 lbah;
> +};
> +struct ata_tf {
> + __u8 dev;
> + __u8 command;
> + __u8 error;
> + __u8 status;
> + __u8 is_lba48;
> + struct ata_lba_regs lob;
> + struct ata_lba_regs hob;
> +};

Don't these things already exist in some standard header file? If not,
shouldn't they be added in a more central location?

> +__u64 tf_to_lba (struct ata_tf *tf)
> +{
> + __u32 lba24, lbah;
> + __u64 lba64;
> +
> + lba24 = (tf->lob.lbah << 16) | (tf->lob.lbam << 8) | (tf->lob.lbal);
> + if (tf->is_lba48)
> + lbah = (tf->hob.lbah << 16) | (tf->hob.lbam << 8) |
> (tf->hob.lbal);
> + else
> + lbah = (tf->dev & 0x0f);
> + lba64 = (((__u64)lbah) << 24) | (__u64)lba24;
> + return lba64;
> +}
> +
> static ssize_t
> sd_store_cache_type(struct device *dev, struct device_attribute *attr,
> const char *buf, size_t count)
> @@ -1839,6 +1892,18 @@ sd_read_cache_type(struct scsi_disk *sdk
> int old_rcd = sdkp->RCD;
> int old_dpofua = sdkp->DPOFUA;
>
> + struct ata_tf tf;
> + struct sg_io_hdr io_hdr;
> + unsigned char cdb[SG_ATA_16_LEN] = {0};
> + unsigned char sb[32] = {0};
> + unsigned char buf[512] = {0};

Arrays generally should not have static initializers. Also, a 512-byte
array is too large to allocate on the stack. And there's already a
512-byte array available -- it's named "buffer".

> + unsigned short wce_word = 0;

There's no reason to initialize this variable.

> + void *data_cmd = buf;

Why do you need to alias a perfectly good variable?

> +
> + memset(cdb, 0, SG_ATA_16_LEN);
> + memset(&tf, 0, sizeof(struct ata_tf));
> + memset(&io_hdr, 0, sizeof(struct sg_io_hdr));

There's no point initializing these things before you know that they
will be used.

> +
> first_len = 4;
> if (sdp->skip_ms_page_8) {
> if (sdp->type == TYPE_RBC)
> @@ -1961,7 +2026,6 @@ Page_found:
> sdkp->DPOFUA ? "supports DPO and FUA"
> : "doesn't support DPO or FUA");
>
> - return;
> }
>
> bad_sense:
> @@ -1974,8 +2038,64 @@ bad_sense:
> sd_printk(KERN_ERR, sdkp, "Asking for cache data failed\n");
>
> defaults:
> - sd_printk(KERN_ERR, sdkp, "Assuming drive cache: write through\n");
> - sdkp->WCE = 0;
> + if (sdkp->WCE)
> + return;
> + else {
> + sd_printk(KERN_ERR, sdkp, "Normal Routine Failed: Trying ATA_16\n");

Instead of adding an awful lot of code inside an "else" clause, it
would be better to put this into its own subroutine.

> +
> + /* The below are necessary parameters which are to set - in order
> + to make use of ATA_OP_IDENTIFY command */
> + tf.lob.lbal = 0;
> + tf.lob.lbam = 0;
> + tf.lob.lbah = 0;
> + tf.lob.nsect = 1; //Number of Sectors to Read
> + tf.lob.feat = 0;
> +
> + /* Command Descriptor Block For SCSI */
> + cdb[0] = SG_ATA_16;
> + cdb[1] = 0x08;
> + cdb[2] = 0x0e;
> + cdb[6] = 0x01; //No. of Sectors To Read
> + cdb[13] = ATA_USING_LBA;
> + cdb[14] = ATA_OP_IDENTIFY;
> +
> + io_hdr.cmd_len = SG_ATA_16_LEN;
> + io_hdr.interface_id = SG_INTERFACE_ID_ORIG;
> + io_hdr.mx_sb_len= sizeof(sb);
> + io_hdr.dxfer_direction = SG_DXFER_FROM_DEV;
> + io_hdr.dxfer_len = sizeof(buf);
> + io_hdr.dxferp = data_cmd;
> + io_hdr.cmdp = cdb;
> + io_hdr.sbp = sb;
> + io_hdr.pack_id = tf_to_lba(&tf);
> + io_hdr.timeout = 0;
> +
> + if (!scsi_cmd_ioctl(sdkp->disk->queue, sdkp->disk,
> O_RDONLY|O_NONBLOCK, SG_IO, &io_hdr))

Do you really need to do an ioctl? Why not call scsi_execute_req()
directly?

> + {
> +#if 0
> +#define DUMP_BYTES_BUFFER(x...) printk( x )
> + int i;
> + for (i = 0; i < 32; i++)
> + DUMP_BYTES_BUFFER(" %02x", sb[i]);
> + DUMP_BYTES_BUFFER("\n");
> + for (i = 0; i < 512; i++)
> + DUMP_BYTES_BUFFER(" %02x", buf[i]);
> + DUMP_BYTES_BUFFER("\n");
> + printk(KERN_ERR"82 - [0x%x], 85 -
> [0x%x]\n",((unsigned short*)data_cmd)[82],((unsigned
> short*)data_cmd)[85]);
> +#endif

For the final patch submission, of course this section should be
removed.

> + /* '6th' Bit in Word 85 Corresponds to Write Cache being Enabled/disabled*/
> + wce_word = le16_to_cpu(((unsigned short*)data_cmd)[85]);
> + if (wce_word & 0x20) {
> + sdkp->WCE = 1;
> + sd_printk(KERN_NOTICE, sdkp, "Write Cache Enabled
> after ATA_16 response\n");
> + } else
> + goto write_through;
> + } else {
> +write_through:
> + sd_printk(KERN_ERR, sdkp, "Assuming drive cache: write through\n");
> + sdkp->WCE = 0;
> + }
> + }
> sdkp->RCD = 0;
> sdkp->DPOFUA = 0;
> }

Besides the potential problems raised by other people, these structural
weaknesses in the patch should be fixed.

Alan Stern



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