Re: [PATCH] scsi: libsas: Reset num_scatter if libata mark qc as NODATA

[John Garry <john.garry@huawei.com>]

On 16/03/2021 19:39, Jolly Shah wrote:
> When the cache_type for the scsi device is changed, the scsi layer
> issues a MODE_SELECT command. The caching mode details are communicated
> via a request buffer associated with the scsi command with data
> direction set as DMA_TO_DEVICE (scsi_mode_select). When this command
> reaches the libata layer, as a part of generic initial setup, libata
> layer sets up the scatterlist for the command using the scsi command
> (ata_scsi_qc_new). This command is then translated by the libata layer
> into ATA_CMD_SET_FEATURES (ata_scsi_mode_select_xlat). The libata layer
> treats this as a non data command (ata_mselect_caching), since it only
> needs an ata taskfile to pass the caching on/off information to the
> device. It does not need the scatterlist that has been setup, so it does
> not perform dma_map_sg on the scatterlist (ata_qc_issue). 

So if we don't perform the dma_map_sg() on the sgl at this point, then 
it seems to me that we should not perform for_each_sg() on it either, 
right? That is still what happens in sas_ata_qc_issue() in this case.

> Unfortunately,
> when this command reaches the libsas layer(sas_ata_qc_issue), libsas
> layer sees it as a non data command with a scatterlist. It cannot
> extract the correct dma length, since the scatterlist has not been
> mapped with dma_map_sg for a DMA operation. When this partially
> constructed SAS task reaches pm80xx LLDD, it results in below warning.
> 
> "pm80xx_chip_sata_req 6058: The sg list address
> start_addr=0x0000000000000000 data_len=0x0end_addr_high=0xffffffff
> end_addr_low=0xffffffff has crossed 4G boundary"
> 
> This patch assigns appropriate value to  num_sectors for ata non data
> commands.
> 
> Signed-off-by: Jolly Shah <jollys@google.com>
> ---
>   drivers/scsi/libsas/sas_ata.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/scsi/libsas/sas_ata.c b/drivers/scsi/libsas/sas_ata.c
> index 024e5a550759..94ec08cebbaa 100644
> --- a/drivers/scsi/libsas/sas_ata.c
> +++ b/drivers/scsi/libsas/sas_ata.c
> @@ -209,10 +209,12 @@ static unsigned int sas_ata_qc_issue(struct ata_queued_cmd *qc)
>   		task->num_scatter = si;
>   	}
>   
> -	if (qc->tf.protocol == ATA_PROT_NODATA)
> +	if (qc->tf.protocol == ATA_PROT_NODATA) {
>   		task->data_dir = DMA_NONE;
> -	else
> +		task->num_scatter = 0;

task->num_scatter has already been set in this function. Best not set it 
twice.

Thanks,
John

> +	} else {
>   		task->data_dir = qc->dma_dir;
> +	}
>   	task->scatter = qc->sg;
>   	task->ata_task.retry_count = 1;
>   	task->task_state_flags = SAS_TASK_STATE_PENDING;
>