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Date:   Wed, 14 Oct 2020 18:44:43 +0200
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Johan Hovold <johan@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-usb@vger.kernel.org,
        Thomas Winischhofer <thomas@winischhofer.net>,
        "Ahmed S. Darwish" <a.darwish@linutronix.de>,
        Mathias Nyman <mathias.nyman@intel.com>,
        Valentina Manea <valentina.manea.m@gmail.com>,
        Shuah Khan <shuah@kernel.org>, linux-omap@vger.kernel.org,
        Kukjin Kim <kgene@kernel.org>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        linux-arm-kernel@lists.infradead.org,
        linux-samsung-soc@vger.kernel.org, Felipe Balbi <balbi@kernel.org>,
        Duncan Sands <duncan.sands@free.fr>
Subject: Re: [patch 03/12] USB: serial: keyspan_pda: Consolidate room query
Message-ID: <20201014164443.husmibyqb4ukw5xg@linutronix.de>
References: <20201014145215.518912759@linutronix.de>
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 <20201014163425.GF712494@rowland.harvard.edu>
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On 2020-10-14 12:34:25 [-0400], Alan Stern wrote:
> On Wed, Oct 14, 2020 at 06:27:14PM +0200, Sebastian Andrzej Siewior wrote:
> > On 2020-10-14 12:14:33 [-0400], Alan Stern wrote:
> > > Instead, consider using the new usb_control_msg_recv() API.  But it 
> > > might be better to allocate the buffer once and for all.
> > 
> > This will still allocate and free buffer on each invocation. What about
> 
> Yes.  That's why I suggesting doing a single buffer allocation at the 
> start and using it for each I/O transfer.  (But I'm not familiar with 
> this code, and I don't know if there might be multiple transfers going 
> on concurrently.)

There are no concurrent transfer. There is a bit used as a lock. The
first one does the transfer, the other wait.

> > moving the query_buf to the begin of the struct / align it?
> 
> No, thank won't work either.  The key to the issue is that while some 
> memory is mapped for DMA, the CPU must not touch it or anything else in 
> the same cache line.  If a field is a member of a data structure, the 
> CPU might very well access a neighboring member while this one is 
> mapped, thereby messing up the cache line.

that is unfortunately true. Let me do the single buffer.

> Alan Stern

Sebastian