Revert "USB: serial: ch341: fix character loss at high transfer rates"

This reverts commit 3c18e9baee0ef97510dcda78c82285f52626764b.

These devices do not appear to send a zero-length packet when the transfer size is a multiple of the bulk-endpoint max-packet size. This means that incoming data may not be processed by the driver until a short packet is received or the receive buffer is full.

Revert back to using endpoint-sized receive buffers to avoid stalled reads.

---
 drivers/usb/serial/ch341.c | 1 -
 1 file changed, 1 deletion(-)

index 8a521b5ea769..2db917eab799 100644
@@ -851,7 +851,6 @@ static struct usb_serial_driver ch341_device = {
 		.owner	= THIS_MODULE,
 		.name	= "ch341-uart",
 	},
-	.bulk_in_size      = 512,
 	.id_table          = id_table,
 	.num_ports         = 1,
 	.open              = ch341_open,

2.31.1

Sorry for this, I didn't notice any issue here (aside for the chip working where it used not to). I have no idea what these zero-length packets correspond to, nor why they're affected by the transfer size. Do you have any idea what I should look for ? Because without that patch, the device is unusable for me :-/

Thanks!
Willy

Zero-length packets are used to indicate completion of bulk transfers that are multiples of the endpoint max-packet size (as per the USB spec). Without those the host controller driver doesn't now that the transfer is complete and that it should call the driver completion callback (and instead waits for the other completion conditions).

It may be possible to configure the device to send ZLPs somehow but since there's no public documentation for the protocol that may require some reverse engineering.

Johan

[ removed Cc stable for the rest of the discussion ]

Thanks for the explanation. I guess that in my case, given that the serial port would emit lots of continuous data (e.g. "find /" or "dmesg"), there's always something pending and the risk that it ends exactly on a 64-byte boundary remained low and never happened in practice.

I totally understand. I'll drop my CH34x adapters and try to figure more suitable ones (i.e. some which work *by default* under Linux). Their small footprint was nice but without doc they're only usable for low speeds :-/

Thanks!
Willy