linux-kernel - Re: [PATCHv2] usb: gadget: storage: optional SCSI WRITE FUA bit

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Date:	Wed, 14 Jul 2010 16:24:23 +0200
From:	Michał Nazarewicz <m.nazarewicz@...sung.com>
To:	Andy Shevchenko <andy.shevchenko@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Andy Shevchenko <ext-andriy.shevchenko@...ia.com>,
	Denis Karpov <ext-denis.2.karpov@...ia.com>,
	Adrian Hunter <adrian.hunter@...ia.com>,
	Alan Stern <stern@...land.harvard.edu>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	Greg Kroah-Hartman <gregkh@...e.de>, linux-usb@...r.kernel.org
Subject: Re: [PATCHv2] usb: gadget: storage: optional SCSI WRITE FUA bit

On Wed, 14 Jul 2010 15:44:58 +0200, Andy Shevchenko <andy.shevchenko@...il.com> wrote:
>>> @@ -93,6 +93,8 @@
>>>  *     removable               Default false, boolean for removable media
>>>  *     luns=N                  Default N = number of filenames, number of
>>>  *                                     LUNs to support
>>> + *     fua=b[,b...]            Default false, booleans for ignore FUA
>>> flag
>>> + *                                     in SCSI WRITE(6,10,12) commands
>>
>> I wonder if it makes sense to make it per-LUN.  I would imagine
>> that it's great to ignore FUA if the device has its own power supply
>> in which case after disconnect the data won't be lost.  This is a
>> per-device property not really per-LUN.  As such I'd make this option
>> global for the gadget.

> Make sense only for removable media with one partition.
> Otherwise. why we have sync option per partition f.e., not per device?

Ah, OK, I see why this is per LUN.  You want to be able not to ignore
FUA if the backing storage is a removable media, right?


>>> +       ssize_t         rc = count;

>> Not really needed here.

> See below

This still stands.  The “rc” is not needed.

>>> +       if (sscanf(buf, "%d", &i) != 1)
>>> +               return -EINVAL;

>> Why not simple_strtol() directly?

> I did it in the same way like fsg_store_ro() does.
> I have no objections to back to previous solution.

OK.  I'd use simpre_strol() myself.  Maybe even patched fsg_store_ro().

>>> +
>>> +       if (curlun->fua)
>>> +               fsg_lun_fsync_sub(curlun);

>> Shouldn't that read something like:
>>
>> +       if (!curlun->fua && i)
>> +               fsg_lun_fsync_sub(curlun);
>>
>> ie. there is no sense in syncing if FUA was active (in which case all
>> writes were synced already, right?) or if the new value is false (since
>> then user does not won't syncing).

> The idea is to sync data before switching from async mode.

But there can be a case of switching from async to async when syncing
is not necessary.  That's why I proposed the &&.  With fua = 1 meaning
ignore the flag my proposal would be:

+       if (!i && curlun->fua)
+               fsg_lun_fsync_sub(curlun);

> Actually fua = 1 means ignorance of that flag.

ignore_fua would be better name then I think.  This also stands for
module parameter.

-- 
Best regards,                                        _     _
| Humble Liege of Serenely Enlightened Majesty of  o' \,=./ `o
| Computer Science,  Michał "mina86" Nazarewicz       (o o)
+----[mina86*mina86.com]---[mina86*jabber.org]----ooO--(_)--Ooo--
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