From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7D8F93F823A; Fri, 10 Jul 2026 12:38:18 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783687099; cv=none; b=Fi0humfMKLbuRVDRo76WwMsQTZ5ATD/QiNXMHGSft/reu4u/TkCUuujpJzXaIPI07AUtMDNvPAcNu8IBmurWtjIeTlNBVtphNQeYQwlgXgifUZJemWFysWkPMjDts+bULiUc5iQVoQ5aCCpGK7CA1oBcGCdsGN75VycXWhhM78Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783687099; c=relaxed/simple; bh=5hn7ZcGD8Dwx44xO43WwgCfQZHXo/1lcuM52V7YTsAs=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=uZu53PGIjeSseco6mTVBpAfdowO7rLfOayk1S2pUq0opD7hT/+wnRP0dbo43lWCvEfRp6KNiJ2I7cZz3uJMjX9Yl3qJwtZkSQi6zRF1l6rV2RrQAglW/TDEaFNKL5n1b18jwC3LUMTh1hlYUI/vVOmJAT1baXStfpRpVaH+9Z7U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=NN6qe6Om; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="NN6qe6Om" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 96A191F000E9; Fri, 10 Jul 2026 12:38:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1783687098; bh=4FUAQD0h6gFwBi/zHnh/dhcRVI8pD9TzsqSXLN+CDu4=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=NN6qe6OmbFlfy3Svxasz6ihKX32T5AWn9rOuZQXgtsDXhXzUze0Ywp2tYBXmg+l7J 0CU6UNUYXfLPnQzESyS/l/DvFANn+k/0fblDLfo6NbLvc9Qh6TMXOIo3bFcbWm3/ET UbnVrKKXoqdhmzNNiD0WxGx2Y2bnvgQyww4x8ymM= Date: Fri, 10 Jul 2026 14:38:14 +0200 From: Greg Kroah-Hartman To: Eric Curtin Cc: Russell King , Jiri Slaby , linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] serial: amba-pl011: don't wait for BUSY after every earlycon character Message-ID: <2026071036-unworn-bunny-fec5@gregkh> References: <20260703191638.131476-1-ericcurtin17@gmail.com> <20260703193805.135511-1-ericcurtin17@gmail.com> Precedence: bulk X-Mailing-List: linux-serial@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260703193805.135511-1-ericcurtin17@gmail.com> On Fri, Jul 03, 2026 at 07:38:04PM +0000, Eric Curtin wrote: > pl011_putc(), used exclusively by the pl011 earlycon (pl011_early_write() > -> uart_console_write()), waits for UART01x_FR_TXFF to clear before > writing a character (correct: don't overrun the TX FIFO) and then *also* > busy-waited for UART01x_FR_BUSY to clear before returning, i.e. it waited > for the character to be fully shifted out on the wire before the next > character in the string could even be considered. > > Waiting for BUSY per character defeats the TX FIFO: instead of letting > the UART buffer several queued bytes and transmit them back to back, > every single character printed through earlycon was forced to wait for > that character's own complete transmission (a full UART bit-time at the > configured baud rate) before the driver would even look at writing the > next one. This is wasted time on real hardware, and it is much worse > under virtualization: each read of UARTFR and each write to UARTDR is > an MMIO access that traps to the hypervisor, so every extra poll is a > full VM-exit/entry round trip. > > git blame on this function is unhelpful (this tree's history stops at a > shallow-clone boundary), but the equivalent history is available from > the upstream patch that added the QDF2400 erratum 44 workaround: So this was generated by a LLM, right? Otherwise why can't you find where this changed by NOT doing a shallow clone? When using a LLM, you must document that. thanks, greg k-h