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From: Rusty Russell <rusty@rustcorp.com.au>
To: "Michael S. Tsirkin" <mst@redhat.com>,
	"Sjur Brændeland" <sjurbren@gmail.com>
Cc: Amit Shah <amit.shah@redhat.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org
Subject: Re: [RFC 1/2] virtio_console: Add support for DMA memory allocation
Date: Thu, 06 Sep 2012 11:34:25 +0930	[thread overview]
Message-ID: <87k3w7j49i.fsf@rustcorp.com.au> (raw)
In-Reply-To: <20120904185541.GB3602@redhat.com>

"Michael S. Tsirkin" <mst@redhat.com> writes:
> On Tue, Sep 04, 2012 at 06:58:47PM +0200, Sjur Brændeland wrote:
>> Hi Michael,
>> 
>> > Exactly. Though if we just fail load it will be much less code.
>> >
>> > Generally, using a feature bit for this is a bit of a problem though:
>> > normally driver is expected to be able to simply ignore
>> > a feature bit. In this case driver is required to
>> > do something so a feature bit is not a good fit.
>> > I am not sure what the right thing to do is.
>> 
>> I see - so in order to avoid the binding between driver and device
>> there are two options I guess. Either make virtio_dev_match() or
>> virtcons_probe() fail. Neither of them seems like the obvious choice.
>> 
>> Maybe adding a check for VIRTIO_CONSOLE_F_DMA_MEM match
>> between device and driver in virtcons_probe() is the lesser evil?
>> 
>> Regards,
>> Sjur
>
> A simplest thing to do is change dev id. rusty?

For generic usage, this is correct.  But my opinion is that fallback on
feature non-ack is quality-of-implementation issue: great to have, but
there are cases where you just want to fail with "you're too old".

And in this case, an old system simply will never work.  So it's a
question of how graceful the failure is.

Can your userspace loader can refuse to proceed if the driver doesn't
ack the bits?  If so, it's simpler than a whole new ID.

Cheers,
Rusty.
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
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WARNING: multiple messages have this Message-ID (diff)
From: Rusty Russell <rusty@rustcorp.com.au>
To: "Michael S. Tsirkin" <mst@redhat.com>,
	"Sjur Brændeland" <sjurbren@gmail.com>
Cc: Amit Shah <amit.shah@redhat.com>,
	linux-kernel@vger.kernel.org, Ohad Ben-Cohen <ohad@wizery.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	virtualization@lists.linux-foundation.org
Subject: Re: [RFC 1/2] virtio_console: Add support for DMA memory allocation
Date: Thu, 06 Sep 2012 11:34:25 +0930	[thread overview]
Message-ID: <87k3w7j49i.fsf@rustcorp.com.au> (raw)
In-Reply-To: <20120904185541.GB3602@redhat.com>

"Michael S. Tsirkin" <mst@redhat.com> writes:
> On Tue, Sep 04, 2012 at 06:58:47PM +0200, Sjur Brændeland wrote:
>> Hi Michael,
>> 
>> > Exactly. Though if we just fail load it will be much less code.
>> >
>> > Generally, using a feature bit for this is a bit of a problem though:
>> > normally driver is expected to be able to simply ignore
>> > a feature bit. In this case driver is required to
>> > do something so a feature bit is not a good fit.
>> > I am not sure what the right thing to do is.
>> 
>> I see - so in order to avoid the binding between driver and device
>> there are two options I guess. Either make virtio_dev_match() or
>> virtcons_probe() fail. Neither of them seems like the obvious choice.
>> 
>> Maybe adding a check for VIRTIO_CONSOLE_F_DMA_MEM match
>> between device and driver in virtcons_probe() is the lesser evil?
>> 
>> Regards,
>> Sjur
>
> A simplest thing to do is change dev id. rusty?

For generic usage, this is correct.  But my opinion is that fallback on
feature non-ack is quality-of-implementation issue: great to have, but
there are cases where you just want to fail with "you're too old".

And in this case, an old system simply will never work.  So it's a
question of how graceful the failure is.

Can your userspace loader can refuse to proceed if the driver doesn't
ack the bits?  If so, it's simpler than a whole new ID.

Cheers,
Rusty.

  reply	other threads:[~2012-09-06  2:04 UTC|newest]

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-09-03 13:51 [RFC 1/2] virtio_console: Add support for DMA memory allocation sjur.brandeland
2012-09-03 13:51 ` [RFC 2/2] virtio_console: Add feature to disable console port sjur.brandeland
2012-09-03 13:51 ` sjur.brandeland
2012-09-03 14:30 ` [RFC 1/2] virtio_console: Add support for DMA memory allocation Michael S. Tsirkin
2012-09-03 14:30   ` Michael S. Tsirkin
2012-09-03 14:57   ` Sjur Brændeland
2012-09-03 14:57     ` Sjur Brændeland
2012-09-03 20:27     ` Michael S. Tsirkin
2012-09-03 20:27       ` Michael S. Tsirkin
2012-09-04  6:07       ` Rusty Russell
2012-09-04  6:07         ` Rusty Russell
2012-09-05 13:00         ` Sjur Brændeland
2012-09-05 13:00           ` Sjur Brændeland
2012-09-05 14:35           ` Michael S. Tsirkin
2012-09-05 14:35             ` Michael S. Tsirkin
2012-09-05 18:15             ` Sjur Brændeland
2012-09-05 18:15               ` Sjur Brændeland
2012-09-05 19:16               ` Michael S. Tsirkin
2012-09-05 19:16                 ` Michael S. Tsirkin
2012-09-07  9:24                 ` Sjur Brændeland
2012-09-07  9:24                   ` Sjur Brændeland
2012-09-04 11:28       ` Sjur Brændeland
2012-09-04 11:28         ` Sjur Brændeland
2012-09-04 13:50         ` Michael S. Tsirkin
2012-09-04 13:50           ` Michael S. Tsirkin
2012-09-04 16:58           ` Sjur Brændeland
2012-09-04 16:58             ` Sjur Brændeland
2012-09-04 18:55             ` Michael S. Tsirkin
2012-09-04 18:55               ` Michael S. Tsirkin
2012-09-06  2:04               ` Rusty Russell [this message]
2012-09-06  2:04                 ` Rusty Russell
2012-09-06  5:27                 ` Michael S. Tsirkin
2012-09-06  5:27                   ` Michael S. Tsirkin
2012-09-10 22:11                   ` Michael S. Tsirkin
2012-09-10 22:11                     ` Michael S. Tsirkin
2012-09-12  6:24                     ` Rusty Russell
2012-09-12  6:24                       ` Rusty Russell
2012-09-07  8:35                 ` Sjur Brændeland
2012-09-07  8:35                   ` Sjur Brændeland
2012-09-16  9:44                 ` [PATCH repost] virtio: don't crash when device is buggy Michael S. Tsirkin
2012-09-16  9:44                   ` Michael S. Tsirkin
2012-09-17  4:27                   ` Rusty Russell
2012-09-17  4:27                     ` Rusty Russell
2012-09-18 22:24                     ` Michael S. Tsirkin
2012-09-18 22:24                       ` Michael S. Tsirkin
2012-09-19  4:10                       ` Rusty Russell
2012-09-19  4:10                       ` Rusty Russell
  -- strict thread matches above, loose matches on Subject: below --
2012-09-03 13:51 [RFC 1/2] virtio_console: Add support for DMA memory allocation sjur.brandeland

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