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

Marc Kleine-Budde
2024-10-25T10:09:00Z

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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?

>
> >
> > > + 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?

Marc

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