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

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Message-ID: <20241025-truthful-honest-newt-c371c8-mkl@pengutronix.de>
Date: Fri, 25 Oct 2024 12:08:13 +0200
From: Marc Kleine-Budde <mkl@...gutronix.de>
To: Ming Yu <a0282524688@...il.com>
Cc: tmyu0@...oton.com, lee@...nel.org, linus.walleij@...aro.org, 
	brgl@...ev.pl, andi.shyti@...nel.org, mailhol.vincent@...adoo.fr, 
	andrew+netdev@...n.ch, davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	pabeni@...hat.com, wim@...ux-watchdog.org, linux@...ck-us.net, jdelvare@...e.com, 
	jic23@...nel.org, lars@...afoo.de, ukleinek@...nel.org, 
	alexandre.belloni@...tlin.com, linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org, 
	linux-i2c@...r.kernel.org, linux-can@...r.kernel.org, netdev@...r.kernel.org, 
	linux-watchdog@...r.kernel.org, linux-hwmon@...r.kernel.org, linux-iio@...r.kernel.org, 
	linux-pwm@...r.kernel.org, linux-rtc@...r.kernel.org
Subject: Re: [PATCH v1 1/9] mfd: Add core driver for Nuvoton NCT6694

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

-- 
Pengutronix e.K.                 | Marc Kleine-Budde          |
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