Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754418AbaF0WBP (ORCPT ); Fri, 27 Jun 2014 18:01:15 -0400 Received: from avon.wwwdotorg.org ([70.85.31.133]:55171 "EHLO avon.wwwdotorg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753943AbaF0WBN (ORCPT ); Fri, 27 Jun 2014 18:01:13 -0400 Message-ID: <53ADE9A2.5020305@wwwdotorg.org> Date: Fri, 27 Jun 2014 16:01:06 -0600 From: Stephen Warren User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0 MIME-Version: 1.0 To: Andrew Bresticker CC: devicetree@vger.kernel.org, linux-doc@vger.kernel.org, "linux-tegra@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , linux-usb@vger.kernel.org, Rob Herring , Pawel Moll , Mark Rutland , Ian Campbell , Kumar Gala , Randy Dunlap , Thierry Reding , Russell King , Linus Walleij , Greg Kroah-Hartman , Mathias Nyman , Grant Likely , Alan Stern , Kishon Vijay Abraham I , Arnd Bergmann Subject: Re: [PATCH v1 6/9] usb: xhci: Add NVIDIA Tegra XHCI host-controller driver References: <1403072180-4944-1-git-send-email-abrestic@chromium.org> <1403072180-4944-7-git-send-email-abrestic@chromium.org> <53AB4F2D.2060208@wwwdotorg.org> <53AC6152.5060908@wwwdotorg.org> In-Reply-To: X-Enigmail-Version: 1.5.2 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/27/2014 03:19 PM, Andrew Bresticker wrote: > On Thu, Jun 26, 2014 at 11:07 AM, Stephen Warren wrote: >> On 06/25/2014 06:06 PM, Andrew Bresticker wrote: >>> On Wed, Jun 25, 2014 at 3:37 PM, Stephen Warren wrote: >>>> On 06/18/2014 12:16 AM, Andrew Bresticker wrote: >>>>> Add support for the on-chip XHCI host controller present on Tegra SoCs. >>>>> >>>>> The driver is currently very basic: it loads the controller with its >>>>> firmware, starts the controller, and is able to service messages sent >>>>> by the controller's firmware. The hardware supports device mode as >>>>> well as runtime power-gating, but support for these is not yet >>>>> implemented here. > >>>>> diff --git a/drivers/usb/host/xhci-tegra.c b/drivers/usb/host/xhci-tegra.c > >>>>> +static int tegra_xhci_set_ss_clk(struct tegra_xhci_hcd *tegra, >>>>> + unsigned long rate) >>>> >>>>> + switch (rate) { >>>>> + case TEGRA_XHCI_SS_CLK_HIGH_SPEED: >>>>> + /* Reparent to PLLU_480M. Set div first to avoid overclocking */ >>>>> + old_parent_rate = clk_get_rate(clk_get_parent(clk)); >>>>> + new_parent_rate = clk_get_rate(tegra->pll_u_480m); >>>>> + div = new_parent_rate / rate; >>>>> + ret = clk_set_rate(clk, old_parent_rate / div); >>>>> + if (ret) >>>>> + return ret; >>>>> + ret = clk_set_parent(clk, tegra->pll_u_480m); >>>>> + if (ret) >>>>> + return ret; >>>> >>>> Don't you need to call clk_set_rate() again after reparenting, since the >>>> divisor will be different, and the rounding too. >>> >>> Nope, the divider we set before remains in-tact after clk_set_parent(). >> >> Oh I see, the clk_set_rate() call is setting up div so it's appropriate >> after the new parent is selected. >> >> Wouldn't it be better to just stop the clock, assert reset, reparent the >> clock, and then set the desired rate directly? > > I'm not sure how that would be better than making it more obvious as > to how we arrive at the final rate. Keep in mind that the XHCI host > is running at this point (we usually get the scale-up message as a > USB3 device is being enumerated) and that disabling the clock and/or > asserting reset to the SS partition clock may not be the best idea... Oh, this happens while the device is running rather than when initializing it? Applying reset is probably a bad idea then. Still, perhaps stopping the clock for a short time is fine? What about: clk_disable_unprepare(clk); clk_set_parent(clk, tegra->pll_u_480m); clk_set_rate(clk, rate); clk_prepare_enable(clk); That seems much more direct to me. The code above feels over-complex to me. If the clock really can't be stopped, then I suppose the existing code in the patch is fine. I'd like to see a final clk_get_rate(clk) call added, and the value compared against the expected value, to make sure no rounding/truncation of the divider happened though. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/