Re: [PATCH 01/11] blk-mq: Add blk_mq_init_queue_ops()

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On 22/03/2022 14:03, Hannes Reinecke wrote:

As mentioned in the cover letter response, it just seems best to keep the normal scsi_cmnd payload but have other means to add on the internal command data, like using host_scribble or scsi_cmnd priv data.

Well; I found that most drivers I had been looking at the scsi command payload isn't used at all; the drivers primarily cared about the (driver-provided) payload, and were completely ignoring the scsi command payload.

Similar for ATA/libsas: you basically never issue real scsi commands, but either 'raw' ATA requests or SCSI TMFs. None of which are scsi commands, so providing them is a bit of a waste.

(And causes irritations, too, as a scsi command requires associated pointers like ->device etc to be set up. Which makes it tricky to use for the initial device setup.)

A problem I see is that in scsi_mq_init_request() we allocate memories like sense_buffer and prot_sdb and store the pointers in the scsi_cmnd payload. If we then reuse a scsi_cmnd payload as an "internal" command payload then this data may be lost.

It might be possible to reuse the scsi cmnd payload for the "internal", but I would rather not get hung up on it now if possible.

Thanks,
John

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