From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Wed, 18 Aug 2021 08:56:44 +0530 From: Viresh Kumar Subject: Re: [virtio-dev] Re: [PATCH V2] virtio: i2c: Allow zero-length transactions Message-ID: <20210818032644.gld65jqwvxjgwqeq@vireshk-i7> References: <20210816104430-mutt-send-email-mst@kernel.org> <20210817035324.r27cdx3yu2j4qmms@vireshk-i7> <20210817024607-mutt-send-email-mst@kernel.org> <20210817104314.yg2ybrodkpnfwpoq@vireshk-i7> MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit To: Jie Deng Cc: "Michael S. Tsirkin" , Cornelia Huck , Wolfram Sang , Paolo Bonzini , Vincent Guittot , Jason Wang , Bill Mills , Alex =?utf-8?Q?Benn=C3=A9e?= , virtio-dev@lists.oasis-open.org, stratos-dev@op-lists.linaro.org, Gerd Hoffmann , arnd@kernel.org List-ID: +Arnd On 18-08-21, 10:38, Jie Deng wrote: > > On 2021/8/17 18:43, Viresh Kumar wrote: > > On 17-08-21, 02:47, Michael S. Tsirkin wrote: > > > On Tue, Aug 17, 2021 at 09:23:24AM +0530, Viresh Kumar wrote: > > > > On 16-08-21, 10:45, Michael S. Tsirkin wrote: > > > > > So I wonder. What if we allow zero-length buffers in virtio? Would that > > > > > address the need? > > > OK, so yea. Let's split this up please. > > Just to make sure I understand what you asked for, we are looking for > > something on these lines ? And the kernel side will still send an 'sg' > > element and call sg_init_one(sg, NULL, 0) ? > > > From the perspective of specification，I think we can allow zero-length > buffers in virtio. > > we can use the len of descriptor to see if it is a zero-length buffer. > > But for a specific implementation, I don't think "NULL" can be passed to > this API. > > There is a check "BUG_ON(!virt_addr_valid(buf))" in it. I tried to look at implementations of virt_addr_valid() and it doesn't check for NULL specifically (for the ones I looked at). I haven't tested it though. Though I am not sure what's better here, remove the need of sending buffer altogether, the way this patch proposed initially or what Michael has suggested. And all that to prevent just a single bit to be used in flags field, which will likely be used for more things later on. -- viresh