Re: [PATCH v1 1/9] mfd: Add core driver for Nuvoton NCT6694

Ming Yu
2024-10-29T03:45:43Z

Marc Kleine-Budde <mkl@pengutronix.de> 於 2024年10月28日 週一 下午10:06寫道：
>
> On 28.10.2024 16:31:25, Ming Yu wrote:
> > > > > > > > > The Linux USB stack can receive bulk messages longer than the max packet size.
> > > > > > > >
> > > > > > > > [Ming] Since NCT6694's bulk pipe endpoint size is 128 bytes for this MFD device.
> > > > > > > > The core will divide packet 256 bytes for high speed USB device, but
> > > > > > > > it is exceeds
> > > > > > > > the hardware limitation, so I am dividing it manually.
> > > > > > >
> > > > > > > You say the endpoint descriptor is correctly reporting it's max packet
> > > > > > > size of 128, but the Linux USB will send packets of 256 bytes?
> > > > > >
> > > > > > [Ming] The endpoint descriptor is correctly reporting it's max packet
> > > > > > size of 256, but the Linux USB may send more than 256 (max is 512)
> > > > > > https://elixir.bootlin.com/linux/v6.11.5/source/drivers/usb/host/xhci-mem.c#L1446
> > > > >
> > > > > AFAIK according to the USB-2.0 spec the maximum packet size for
> > > > > high-speed bulk transfers is fixed set to 512 bytes. Does this mean that
> > > > > your device is a non-compliant USB device?
> > > >
> > > > We will reduce the endpoint size of other interfaces to ensure that MFD device
> > > > meets the USB2.0 spec. In other words, I will remove the code for manual
> > > > unpacking in the next patch.
> > >
> > > I was not talking about the driver, but your USB device. According to
> > > the USB2.0 spec, the packet size is fixed to 512 for high-speed bulk
> > > transfers. So your device must be able to handle 512 byte transfers or
> > > it's a non-compliant USB device.
> >
> > I understand. Therefore, the USB device's firmware will be modified to support
> > bulk pipe size of 512 bytes to comply with the USB 2.0 spec.
>
> Then you don't need manual segmentation of bulk transfers anymore!

Understood, thank you very much.

>
> > > > > > > > > > + for (i = 0, len = length; len > 0; i++, len -= packet_len) {
> > > > > > > > > > + if (len > nct6694->maxp)
> > > > > > > > > > + packet_len = nct6694->maxp;
> > > > > > > > > > + else
> > > > > > > > > > + packet_len = len;
> > > > > > > > > > +
> > > > > > > > > > + ret = usb_bulk_msg(udev, usb_rcvbulkpipe(udev, BULK_IN_ENDPOINT),
> > > > > > > > > > + nct6694->rx_buffer + nct6694->maxp * i,
> > > > > > > > > > + packet_len, &rx_len, nct6694->timeout);
> > > > > > > > > > + if (ret)
> > > > > > > > > > + goto err;
> > > > > > > > > > + }
> > > > > > > > > > +
> > > > > > > > > > + for (i = 0; i < rd_len; i++)
> > > > > > > > > > + buf[i] = nct6694->rx_buffer[i + rd_idx];
> > > > > > > > >
> > > > > > > > > memcpy()?
> > > > > > > > >
> > > > > > > > > Or why don't you directly receive data into the provided buffer? Copying
> > > > > > > > > of the data doesn't make it faster.
> > > > > > > > >
> > > > > > > > > On the other hand, receiving directly into the target buffer means the
> > > > > > > > > target buffer must not live on the stack.
> > > > > > > >
> > > > > > > > [Ming] Okay! I'll change it to memcpy().
> > > > > > >
> > > > > > > fine!
> > > > > > >
> > > > > > > > This is my perspective: the data is uniformly received by the rx_bffer held
> > > > > > > > by the MFD device. does it need to be changed?
> > > > > > >
> > > > > > > My question is: Why do you first receive into the nct6694->rx_buffer and
> > > > > > > then memcpy() to the buffer provided by the caller, why don't you
> > > > > > > directly receive into the memory provided by the caller?
> > > > > >
> > > > > > [Ming] Due to the bulk pipe maximum packet size limitation, I think consistently
> > > > > > using the MFD'd dynamically allocated buffer to submit URBs will better
> > > > > > manage USB-related operations
> > > > >
> > > > > The non-compliant max packet size limitation is unrelated to the
> > > > > question which RX or TX buffer to use.
> > > >
> > > > I think these two USB functions can be easily called using the buffer
> > > > dynamically
> > > > allocated by the MFD. However, if they transfer data directly to the
> > > > target buffer,
> > > > they must ensure that it is not located on the stack.
> > >
> > > You have a high coupling between the MFD driver and the individual
> > > drivers anyways, so why not directly use the dynamically allocated
> > > buffer provided by the caller and get rid of the memcpy()?
> >
> > Okay! I will provide a function to request and free buffer for child devices,
> > and update the caller's variables to use these two functions in the next patch.
>
> I don't see a need to provide dedicated function to allocate and free
> the buffers. The caller can allocate them as part of their private data,
> or allocate them during probe().

Okay, so each child device may allocate a buffer like this during probe():
priv->xmit_buf = devm_kcalloc(dev, MAX_PACKET_SZ, sizeof(unsigned char),
GFP_KERNEL), right?

>
> regards,
> Marc
>
> --
> Pengutronix e.K. | Marc Kleine-Budde |
> Embedded Linux | https://www.pengutronix.de |
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Thanks,
Ming