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

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Message-ID: <AANLkTil1eG5yzBxvryf-TZINYjeGph-rNz8eELQtm5tj@mail.gmail.com>
Date:	Wed, 14 Jul 2010 16:44:58 +0300
From:	Andy Shevchenko <andy.shevchenko@...il.com>
To:	Michał Nazarewicz <m.nazarewicz@...sung.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

2010/7/14 Michał Nazarewicz <m.nazarewicz@...sung.com>:
> On Wed, 14 Jul 2010 11:05:31 +0200, Andy Shevchenko
> <andy.shevchenko@...il.com> wrote:
>>
>> MS Windows mounts removable storage in "Removal optimized mode" by
>> default. All the writes to the media are synchronous which is achieved
>> by setting FUA (Force Unit Access) bit in SCSI WRITE(10,12) commands.
>> This prevents I/O requests aggregation in block layer dramatically
>> decreasing performance.
>
>> diff --git a/drivers/usb/gadget/file_storage.c
>> b/drivers/usb/gadget/file_storage.c
>> index b49d86e..45f58d9 100644
>> --- a/drivers/usb/gadget/file_storage.c
>> +++ b/drivers/usb/gadget/file_storage.c
>> @@ -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?

>> @@ -743,6 +752,24 @@ static ssize_t fsg_store_ro(struct device *dev,
>> struct device_attribute *attr,
>>        return rc;
>>  }
>> +static ssize_t fsg_store_fua(struct device *dev, struct device_attribute
>> *attr,
>> +                            const char *buf, size_t count)
>> +{
>> +       ssize_t         rc = count;
>
> Not really needed here.
See below

>
>> +       struct fsg_lun  *curlun = fsg_lun_from_dev(dev);
>> +       int             i;
>> +
>> +       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.

>> +
>> +       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.
Actually fua = 1 means ignorance of that flag.


-- 
With Best Regards,
Andy Shevchenko
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