Message-ID: <20241028-observant-gentle-doberman-0a2baa-mkl@pengutronix.de>
Date: Mon, 28 Oct 2024 15:06:39 +0100
From: Marc Kleine-Budde <mkl@...gutronix.de>
To: Ming Yu <a0282524688@...il.com>
Cc: tmyu0@...oton.com, lee@...nel.org, linus.walleij@...aro.org, 
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Subject: Re: [PATCH v1 1/9] mfd: Add core driver for Nuvoton NCT6694

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!

> > > > > > > > > +     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().

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde          |
Embedded Linux                   | https://www.pengutronix.de |
Vertretung Nürnberg              | Phone: +49-5121-206917-129 |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-9   |

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