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

Ming Yu
2024-10-28T08:31:37Z

Marc Kleine-Budde <mkl@pengutronix.de> 於 2024年10月28日 週一 下午3:52寫道：
>
> On 28.10.2024 15:33:08, Ming Yu wrote:
> > Dear Marc,
> >
> > Thank you for your comments,
> >
> > Marc Kleine-Budde <mkl@pengutronix.de> 於 2024年10月25日 週五 下午8:24寫道：
> > >
> > > On 25.10.2024 19:03:55, Ming Yu wrote:
> > > > Oh! I'm sorry about that I confused the packet size.
> > > > The NCT6694 bulk maximum packet size is 256 bytes,
> > > > and USB High speed bulk maximum packet size is 512 bytes.
> > > >
> > > > Marc Kleine-Budde <mkl@pengutronix.de> 於 2024年10月25日 週五 下午6:08寫道：
> > > > >
> > > > > On 25.10.2024 16:08:10, Ming Yu wrote:
> > > > > > > > +int nct6694_read_msg(struct nct6694 *nct6694, u8 mod, u16 offset, u16 length,
> > > > > > > > + u8 rd_idx, u8 rd_len, unsigned char *buf)
> > > > > > >
> > > > > > > why not make buf a void *?
> > > > > >
> > > > > > [Ming] I'll change the type in the next patch.
> > > > > >
> > > > > > >
> > > > > > > > +{
> > > > > > > > + struct usb_device *udev = nct6694->udev;
> > > > > > > > + unsigned char err_status;
> > > > > > > > + int len, packet_len, tx_len, rx_len;
> > > > > > > > + int i, ret;
> > > > > > > > +
> > > > > > > > + mutex_lock(&nct6694->access_lock);
> > > > > > > > +
> > > > > > > > + /* Send command packet to USB device */
> > > > > > > > + nct6694->cmd_buffer[REQUEST_MOD_IDX] = mod;
> > > > > > > > + nct6694->cmd_buffer[REQUEST_CMD_IDX] = offset & 0xFF;
> > > > > > > > + nct6694->cmd_buffer[REQUEST_SEL_IDX] = (offset >> 8) & 0xFF;
> > > > > > > > + nct6694->cmd_buffer[REQUEST_HCTRL_IDX] = HCTRL_GET;
> > > > > > > > + nct6694->cmd_buffer[REQUEST_LEN_L_IDX] = length & 0xFF;
> > > > > > > > + nct6694->cmd_buffer[REQUEST_LEN_H_IDX] = (length >> 8) & 0xFF;
> > > > > > > > +
> > > > > > > > + ret = usb_bulk_msg(udev, usb_sndbulkpipe(udev, BULK_OUT_ENDPOINT),
> > > > > > > > + nct6694->cmd_buffer, CMD_PACKET_SZ, &tx_len,
> > > > > > > > + nct6694->timeout);
> > > > > > > > + if (ret)
> > > > > > > > + goto err;
> > > > > > > > +
> > > > > > > > + /* Receive response packet from USB device */
> > > > > > > > + ret = usb_bulk_msg(udev, usb_rcvbulkpipe(udev, BULK_IN_ENDPOINT),
> > > > > > > > + nct6694->rx_buffer, CMD_PACKET_SZ, &rx_len,
> > > > > > > > + nct6694->timeout);
> > > > > > > > + if (ret)
> > > > > > > > + goto err;
> > > > > > > > +
> > > > > > > > + err_status = nct6694->rx_buffer[RESPONSE_STS_IDX];
> > > > > > > > +
> > > > > > > > + /*
> > > > > > > > + * Segmented reception of messages that exceed the size of USB bulk
> > > > > > > > + * pipe packets.
> > > > > > > > + */
> > > > > > >
> > > > > > > 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.

>
> > > > > > > > + 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.

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