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Message-ID: <f8c176d4-74f0-3e4f-446f-2a5f8ace3b28@kernel.dk>
Date:   Mon, 14 Mar 2022 14:29:20 -0600
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Subject: Re: [PATCH] pata_parport: add driver (PARIDE replacement)
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To:     Ondrej Zary <linux@zary.sk>
Cc:     Damien Le Moal <damien.lemoal@opensource.wdc.com>,
        Christoph Hellwig <hch@lst.de>, Tim Waugh <tim@cyberelk.net>,
        linux-block@vger.kernel.org, linux-parport@lists.infradead.org,
        linux-ide@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20220312144415.20010-1-linux@zary.sk>
 <202203132015.18183.linux@zary.sk>
 <5161ed17-5f55-e851-c2e2-5340cc62fa3b@kernel.dk>
 <202203142125.40532.linux@zary.sk>
From:   Jens Axboe <axboe@kernel.dk>
In-Reply-To: <202203142125.40532.linux@zary.sk>
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On 3/14/22 2:25 PM, Ondrej Zary wrote:
> On Monday 14 March 2022 00:19:30 Jens Axboe wrote:
>> On 3/13/22 1:15 PM, Ondrej Zary wrote:
>>> On Saturday 12 March 2022 15:44:15 Ondrej Zary wrote:
>>>> The pata_parport is a libata-based replacement of the old PARIDE
>>>> subsystem - driver for parallel port IDE devices.
>>>> It uses the original paride low-level protocol drivers but does not
>>>> need the high-level drivers (pd, pcd, pf, pt, pg). The IDE devices
>>>> behind parallel port adapters are handled by the ATA layer.
>>>>
>>>> This will allow paride and its high-level drivers to be removed.
>>>>
>>>> paride and pata_parport are mutually exclusive because the compiled
>>>> protocol drivers are incompatible.
>>>>
>>>> Tested with Imation SuperDisk LS-120 and HP C4381A (both use EPAT
>>>> chip).
>>>>
>>>> Note: EPP-32 mode is buggy in EPAT - and also in all other protocol
>>>> drivers - they don't handle non-multiple-of-4 block transfers
>>>> correctly. This causes problems with LS-120 drive.
>>>> There is also another bug in EPAT: EPP modes don't work unless a 4-bit
>>>> or 8-bit mode is used first (probably some initialization missing?).
>>>> Once the device is initialized, EPP works until power cycle.
>>>>
>>>> So after device power on, you have to:
>>>> echo "parport0 epat 0" >/sys/bus/pata_parport/new_device
>>>> echo pata_parport.0 >/sys/bus/pata_parport/delete_device
>>>> echo "parport0 epat 4" >/sys/bus/pata_parport/new_device
>>>> (autoprobe will initialize correctly as it tries the slowest modes
>>>> first but you'll get the broken EPP-32 mode)
>>>
>>> Found a bug - the same device can be registered multiple times. Fix
>>> will be in v2. But this revealed a bigger problem: pi_connect can
>>> sleep (uses parport_claim_or_block) and libata does not like that. Any
>>> ideas how to fix this?
>>
>> I think you'd need two things here:
>>
>> - The blk-mq queue should be registered with BLK_MQ_F_BLOCKING, which
>>   will allow blocking off the queue_rq path.
> 
> My knowledge about blk-mq is exactly zero. After grepping the code, I
> guess that BLK_MQ_F_BLOCKING should be used by the block device
> drivers - sd and sr?

The controller would set

->needs_blocking_queue_rq = true;

or something, and we'd default to false. And if that is set, when the
blk-mq queue is created, then we'd set BLK_MQ_F_BLOCKING upon creation
if that flag is true.

That's the block layer side. Then in libata you'd need to ensure that
you check that same setting and invoke ata_qc_issue() appropriately.

Very top level stuff, there might be more things lurking below. But
you'll probably find them as you test this stuff...

-- 
Jens Axboe