Re: [PATCH v2] mpt3sas: Force request partial completion alignment

["Guilherme G. Piccoli" <gpiccoli@linux.vnet.ibm.com>]

On 01/26/2017 03:02 PM, Ram Pai wrote:
> On Thu, Jan 26, 2017 at 11:31:53AM -0200, Guilherme G. Piccoli wrote:
>> On 01/25/2017 09:46 PM, Martin K. Petersen wrote:
>>>>>>>> "Guilherme" == Guilherme G Piccoli <gpiccoli@linux.vnet.ibm.com> writes:
>>>
>>> Hi Guilherme,
>>
>> Hi Martin, thanks for the review!
>>
>>
>>>
>>> diff --git a/drivers/scsi/mpt3sas/mpt3sas_scsih.c b/drivers/scsi/mpt3sas/mpt3sas_scsih.c
>>> index 75f3fce..e52c942 100644
>>> --- a/drivers/scsi/mpt3sas/mpt3sas_scsih.c
>>> +++ b/drivers/scsi/mpt3sas/mpt3sas_scsih.c
>>> @@ -4657,6 +4657,8 @@ _scsih_io_done(struct MPT3SAS_ADAPTER *ioc, u16 smid, u8 msix_index, u32 reply)
>>>  	struct MPT3SAS_DEVICE *sas_device_priv_data;
>>>  	u32 response_code = 0;
>>>  	unsigned long flags;
>>> +	unsigned int sector_sz;
>>> +	struct request *req;
>>>
>>>  	mpi_reply = mpt3sas_base_get_reply_virt_addr(ioc, reply);
>>>  	scmd = _scsih_scsi_lookup_get_clear(ioc, smid);
>>> @@ -4715,6 +4717,21 @@ _scsih_io_done(struct MPT3SAS_ADAPTER *ioc, u16 smid, u8 msix_index, u32 reply)
>>>  	}
>>>
>>>  	xfer_cnt = le32_to_cpu(mpi_reply->TransferCount);
>>> +
>>> +	/* In case of bogus fw or device, we could end up having
>>> +	 * unaligned partial completion. We can force alignment here,
>>> +	 * then scsi-ml does not need to handle this misbehavior.
>>> +	 */
>>> +	sector_sz = scmd->device->sector_size;
>>> +	req = scmd->request;
>>> +	if (unlikely(sector_sz && req && (req->cmd_type == REQ_TYPE_FS) &&
>>> +		    (xfer_cnt % sector_sz))) {
>>>
>>> Maybe a bit zealous on the sanity checking...
>>
>> A bit...? heheh
>> Too much I'd say. Since this is dealing with a bogus FW scenario, I
>> found more safe to check everything...of course we can remove checks if
>> it's sure req isn't NULL ever. The sector_sz check is avoiding
>> degenerate cases, since our division below.
>>
>>
>>>
>>> +		sdev_printk(KERN_INFO, scmd->device,
>>> +			"unaligned partial completion avoided (xfer_cnt=%u, sector_sz=%u)\n",
>>> +			xfer_cnt, sector_sz);
>>> +		xfer_cnt = (xfer_cnt / sector_sz) * sector_sz;
>>>
>>> Not so keen on divisions. xfer_cnt = round_down(xfer_cnt, sector_sz), maybe?
>>>
>>
>> Martin, I might be completely wrong here (please correct me if this is
>> the case), but isn't C standard integer division a truncation that acts
>> like a round down? I checked (what I think is) the specification of C
>> language (ISO/IEC 9899:1999), and it seems the division proposed by Ram
>> Pai is accurate in this case. Also, both variables are unsigned.
> 
> Guilherme,  Its better to use round_down() instead of division. Among
> other things it saves a few nanoseconds.

Thanks Ram and Martin for the suggestion and explanation. I just sent a V3.

Cheers,


Guilherme

> 
> RP
>