Date: Thu, 22 May 2025 11:02:24 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: mmel@FreeBSD.org, src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: Re: 0d2fd5b99c95 - main - ns8250: use LSR_THRE instead of LSR_TEMT for checking tx flush Message-ID: <e9340bb2-754e-44e7-8928-b8331247fe8b@FreeBSD.org> In-Reply-To: <13b1bbfe-4a60-4c4a-aa4e-ed168cb1286a@FreeBSD.org> References: <202505201457.54KEvD1r053951@gitrepo.freebsd.org> <1a11f640-be62-4f4e-b537-70806ac54831@FreeBSD.org> <bc1afdab-72d6-43c1-9603-1eed07cb25e6@FreeBSD.org> <37BF51C0-C631-493F-B3AF-3AA9FC32551B@panasas.com> <13b1bbfe-4a60-4c4a-aa4e-ed168cb1286a@FreeBSD.org>
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On 22/05/2025 09:37, mmel@FreeBSD.org wrote: > I wrote: >> Maybe I don't understand the code, but UART_FLUSH_RECEIVER in ns8250_bus_attach >> looks strange to me. It's one thing to flush data while in the loop-back mode, >> but I think that in ns8250_bus_attach the hardware is fully set up to receive >> data from the outside world. So, how can we hope to drain all of it and to >> reliably detect whether FIFO flushing works. I mean that something on the other >> end could be continuously transmitting. >> > I'm not sure if I understand you exactly, but in loop-back mode the RX input is > internally connected to the TX output (i.e. disconnected from the outside > world). So from that perspective, the FIFO detection and flush seems fine to me. Right. Loop-back mode is used in probe, but in attach the UART is in normal mode as far as I can tell. -- Andriy Gapon
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