linux-kernel - Re: [dpdk-dev] [PATCH 0/2] uio

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Message-ID: <5611589F.3010002@cloudius-systems.com>
Date:	Sun, 4 Oct 2015 19:49:35 +0300
From:	Vlad Zolotarov <vladz@...udius-systems.com>
To:	Alexander Duyck <alexander.duyck@...il.com>,
	Stephen Hemminger <stephen@...workplumber.org>
Cc:	dev@...k.org, Avi Kivity <avi@...lladb.com>, hjk@...sjkoch.de,
	gregkh@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [dpdk-dev] [PATCH 0/2] uio_msi: device driver

FYI: I've just posted to linux-kernel list patches that add support for 
both MSI and MSI-X interrupt modes to uio_pci_generic driver.
It addresses most (all) remarks on this thread and also fixes some 
issues this code has, e.g. not disabling msi-x in remove(), etc.

U are all welcome to comment... ;)

thanks,
vlad

On 10/02/15 04:39, Alexander Duyck wrote:
> On 10/01/2015 05:04 PM, Stephen Hemminger wrote:
>> On Thu, 1 Oct 2015 16:43:23 -0700
>> Alexander Duyck <alexander.duyck@...il.com> wrote:
>>
>>> Yes, but in the case of something like a VF it is going to just make a
>>> bigger mess of things since INTx doesn't work.  So what would you 
>>> expect
>>> your driver to do in that case?  Also we have to keep in mind that the
>>> MSI-X failure case is very unlikely.
>>>
>>> One other thing that just occurred to me is that you may want to try
>>> using the range allocation call instead of a hard set number of
>>> interrupts.  Then if you start running short on vectors you don't hard
>>> fail and instead just allocate what you can.
>> I tried that but the bookkeeping gets messy since there is no good
>> way to communicate that back to userspace and have it adapt.
>
> Actually I kind of just realized that uio_msi_open is kind of messed 
> up.  So if the MSI-X allocation fails due to no resources it will 
> return a positive value indicating the number of vectors that could be 
> allocated, a negative value if one of the input values is invalid, or 
> 0.  I'm not sure if returning a positive value on failure is an issue 
> or not.  I know the open call is supposed to return a negative value 
> or the file descriptor if not negative.  I don't know if the return 
> value might be interpreted as a file descriptor or not.
>
> Also if MSI-X is supported by the hardware, but disabled for some 
> reason by the kernel ("pci=nomsi")  then this driver is rendered 
> inoperable since it will never give you anything but -EINVAL from the 
> open call.
>
> I really think you should probably look at taking care of enabling 
> MSI-X and maybe MSI as a fall-back in probe.  At least then you can 
> post a message about how many vectors are enabled and what type. Then 
> if you cannot enable any interrupts due to MSI being disabled you can 
> simply fail at probe time and let then load a different driver.
>
> - Alex

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