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

[Ming Yu <a0282524688@xxxxxxxxx>]

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@xxxxxxxxxxxxxx> 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

>
> >
> > >
> > > > +     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

>
> Marc
>
> --
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