linux-kernel - Re: [patch 03/12] USB: serial: keyspan

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Message-ID: <87ft6gbx3k.fsf@nanos.tec.linutronix.de>
Date:   Wed, 14 Oct 2020 18:17:51 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Johan Hovold <johan@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org,
        Thomas Winischhofer <thomas@...ischhofer.net>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Mathias Nyman <mathias.nyman@...el.com>,
        Valentina Manea <valentina.manea.m@...il.com>,
        Shuah Khan <shuah@...nel.org>, linux-omap@...r.kernel.org,
        Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, Felipe Balbi <balbi@...nel.org>,
        Duncan Sands <duncan.sands@...e.fr>
Subject: Re: [patch 03/12] USB: serial: keyspan_pda: Consolidate room query

On Wed, Oct 14 2020 at 12:14, Alan Stern wrote:
> On Wed, Oct 14, 2020 at 04:52:18PM +0200, Thomas Gleixner wrote:
>> From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
>> 
>> Having two copies of the same code doesn't make the code more readable and
>> allocating a buffer of 1 byte for a synchronous operation is a pointless
>> exercise.
>
> Not so.  In fact, it is required, because a portion of a structure 
> cannot be mapped for DMA unless it is aligned at a cache line boundary.
>
>> Add a byte buffer to struct keyspan_pda_private which can be used
>> instead. The buffer is only used in open() and tty->write().
>
> This won't work.

Ok.

>> +	res = usb_control_msg(serial->dev, usb_rcvctrlpipe(serial->dev, 0),
>> +			      6, /* write_room */
>> +			      USB_TYPE_VENDOR | USB_RECIP_INTERFACE | USB_DIR_IN,
>> +			      0, /* value */
>> +			      0, /* index */
>> +			      &priv->query_buf,
>> +			      1,
>> +			      2000);
>
> Instead, consider using the new usb_control_msg_recv() API.  But it 
> might be better to allocate the buffer once and for all.

Let me have a look.

Thanks,

        tglx