Messages in this thread | | | Subject | Re: [PATCH v10] i2c: virtio: add a virtio i2c frontend driver | From | Jie Deng <> | Date | Tue, 6 Jul 2021 09:50:23 +0800 |
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On 2021/7/5 20:18, Viresh Kumar wrote: > On 29-06-21, 12:43, Wolfram Sang wrote: >>> From the spec: >>> >>> The case when ``length of \field{write_buf}''=0, and at the same time, >>> ``length of \field{read_buf}''=0 doesn't make any sense. >>> >>> I mentioned this in my first reply and to my understanding I did not get >>> a reply that this has changed meanwhile. >>> >> Also, this code as mentioned before: >> >>> + if (!msgs[i].len) >>> + break; >> I hope this can extended in the future to allow zero-length messages. If >> this is impossible we need to set an adapter quirk instead. > Wolfram, > > I stumbled again upon this while working at the backend implementation. > > If you look at i2c_smbus_xfer_emulated(), the command is always sent via > msgbuf0[0]. Even in the case of I2C_SMBUS_QUICK, where we set msg[0].len = 0, we > still send the buf. This is really confusing :( > > Do I understand correctly that we always need to send msg[0].buf even when > msg[0].len is 0 ? > > If so, it would be difficult to implement this with the current i2c virtio > specification, as the msg.len isn't really passed from guest to host, rather it > is inferred using the length of the buffer itself. And so we can't really pass a > buffer if length is 0. > > Moreover, the driver uses i2c_get_dma_safe_msg_buf(), which also depends on the > length parameter here to allocate the buffer and copy data to it. > > All in all, the latest version of the driver doesn't work with "i2cdetect -q <bus>". > > To make it work, I had to add this: > > diff --git a/drivers/i2c/busses/i2c-virtio.c b/drivers/i2c/busses/i2c-virtio.c > index 731267d42292..5b8bd98ae38e 100644 > --- a/drivers/i2c/busses/i2c-virtio.c > +++ b/drivers/i2c/busses/i2c-virtio.c > @@ -73,6 +73,9 @@ static int virtio_i2c_prepare_reqs(struct virtqueue *vq, > sg_init_one(&out_hdr, &reqs[i].out_hdr, sizeof(reqs[i].out_hdr)); > sgs[outcnt++] = &out_hdr; > > + if (!msgs[i].len) > + msgs[i].len = 1; > + > if (msgs[i].len) { > reqs[i].buf = i2c_get_dma_safe_msg_buf(&msgs[i], 1); > if (!reqs[i].buf) > > which made it I2C_SMBUS_BYTE instead of I2C_SMBUS_QUICK. > > What should we do here Wolfram? > > > Jie, while wolfram comes back and replies to this, I think you need to switch > back to NOT supporting zero length transfer and set update virtio_i2c_func() to > return: > > I2C_FUNC_I2C | (I2C_FUNC_SMBUS_EMUL & ~I2C_FUNC_SMBUS_QUICK); > > Support for zero-length transfers and I2C_FUNC_SMBUS_QUICK can be added > separately. > > Thanks.
It's OK to me. Let's see what Wolfram says when he comes back.
I will send the updated version then.
Thanks.
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