From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 553E2C433EF for ; Mon, 11 Apr 2022 19:54:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232307AbiDKT44 (ORCPT ); Mon, 11 Apr 2022 15:56:56 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45994 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238175AbiDKT4z (ORCPT ); Mon, 11 Apr 2022 15:56:55 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6A6171DA43; Mon, 11 Apr 2022 12:54:38 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 6253BB81896; Mon, 11 Apr 2022 19:54:37 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C01EBC385A3; Mon, 11 Apr 2022 19:54:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1649706876; bh=bOc/33Z7+1jn2eHnVDtC0HCa/hvdIrrV1vtEJE1e728=; h=Date:From:To:Cc:Subject:In-Reply-To:From; b=V6+ZhQQJ0FpA3JTLhfNdMXpuSewBXjtopDEltHhlXbZSpO78QxEP1ZcpA53xrUGsh JZIgs+nnioj5B5rZ6ZYyOhb7PDCoD7KmnK/a9wN3yFN+gEevKq5C7FKAGKVSdCXq5Y OyOxDtW+tcvHx+oApyxc1n3O0GdcPQcKVQnC4aO+t4AYiy/7D5M4rDnBXkLUpIdGcF Mn1DGUs5kOloLRJHvN8s17W01fSA3P9CNLtNXxfP1Wp5sxhX29Q5fOgOnp+bAuGyJV H1WckifF0+jHHfVPXfJsIFHHTscgId3sxp4qkX7Zoj7qKahszAXlbLGHQ5S4ktxgJV SPyfAAuKNR0Sw== Date: Mon, 11 Apr 2022 14:54:34 -0500 From: Bjorn Helgaas To: Pali =?iso-8859-1?Q?Roh=E1r?= Cc: Lorenzo Pieralisi , Bjorn Helgaas , Rob Herring , Andrew Lunn , Thomas Petazzoni , Krzysztof =?utf-8?Q?Wilczy=C5=84ski?= , Marek =?iso-8859-1?Q?Beh=FAn?= , Russell King , linux-pci@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 3/4] PCI: Add function for parsing 'slot-power-limit-milliwatt' DT property Message-ID: <20220411195434.GA531670@bhelgaas> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20220411111407.7ycuoldxjvqnkoo4@pali> Precedence: bulk List-ID: X-Mailing-List: linux-pci@vger.kernel.org On Mon, Apr 11, 2022 at 01:14:07PM +0200, Pali Rohár wrote: > On Friday 08 April 2022 10:27:50 Bjorn Helgaas wrote: > > On Fri, Mar 25, 2022 at 10:38:26AM +0100, Pali Rohár wrote: > > > Add function of_pci_get_slot_power_limit(), which parses the > > > 'slot-power-limit-milliwatt' DT property, returning the value in > > > milliwatts and in format ready for the PCIe Slot Capabilities Register. > > > > > > Signed-off-by: Pali Rohár > > > Signed-off-by: Marek Behún > > > Reviewed-by: Rob Herring > > > --- > > > Changes in v3: > > > * Set 600 W when DT slot-power-limit-milliwatt > 600 W > > > Changes in v2: > > > * Added support for PCIe 6.0 slot power limit encodings > > > * Round down slot power limit value > > > --- > > > drivers/pci/of.c | 64 +++++++++++++++++++++++++++++++++++++++++++++++ > > > drivers/pci/pci.h | 15 +++++++++++ > > > 2 files changed, 79 insertions(+) > > > > > > diff --git a/drivers/pci/of.c b/drivers/pci/of.c > > > index cb2e8351c2cc..5ebff26edd41 100644 > > > --- a/drivers/pci/of.c > > > +++ b/drivers/pci/of.c > > > @@ -633,3 +633,67 @@ int of_pci_get_max_link_speed(struct device_node *node) > > > return max_link_speed; > > > } > > > EXPORT_SYMBOL_GPL(of_pci_get_max_link_speed); > > > + > > > +/** > > > + * of_pci_get_slot_power_limit - Parses the "slot-power-limit-milliwatt" > > > + * property. > > > + * > > > + * @node: device tree node with the slot power limit information > > > + * @slot_power_limit_value: pointer where the value should be stored in PCIe > > > + * Slot Capabilities Register format > > > + * @slot_power_limit_scale: pointer where the scale should be stored in PCIe > > > + * Slot Capabilities Register format > > > + * > > > + * Returns the slot power limit in milliwatts and if @slot_power_limit_value > > > + * and @slot_power_limit_scale pointers are non-NULL, fills in the value and > > > + * scale in format used by PCIe Slot Capabilities Register. > > > + * > > > + * If the property is not found or is invalid, returns 0. > > > + */ > > > +u32 of_pci_get_slot_power_limit(struct device_node *node, > > > + u8 *slot_power_limit_value, > > > + u8 *slot_power_limit_scale) > > > +{ > > > + u32 slot_power_limit_mw; > > > + u8 value, scale; > > > + > > > + if (of_property_read_u32(node, "slot-power-limit-milliwatt", > > > + &slot_power_limit_mw)) > > > + slot_power_limit_mw = 0; > > > + > > > + /* Calculate Slot Power Limit Value and Slot Power Limit Scale */ > > > + if (slot_power_limit_mw == 0) { > > > + value = 0x00; > > > + scale = 0; > > > + } else if (slot_power_limit_mw <= 255) { > > > + value = slot_power_limit_mw; > > > + scale = 3; > > > + } else if (slot_power_limit_mw <= 255*10) { > > > + value = slot_power_limit_mw / 10; > > > + scale = 2; > > > + } else if (slot_power_limit_mw <= 255*100) { > > > + value = slot_power_limit_mw / 100; > > > + scale = 1; > > > + } else if (slot_power_limit_mw <= 239*1000) { > > > + value = slot_power_limit_mw / 1000; > > > + scale = 0; > > > + } else if (slot_power_limit_mw <= 250*1000) { > > > + value = 0xF0; > > > + scale = 0; > > > > I think the spec is poorly worded here. PCIe r6.0, sec 7.5.3.9, says: > > > > F0h > 239 W and <= 250 W Slot Power Limit > > > > I don't think it's meaningful for the spec to include a range here. > > The amount of power the slot can supply has a single maximum. I > > suspect the *intent* of F0h/00b is that a device in the slot may > > consume up to 250W. > > > > Your code above would mean that slot_power_limit_mw == 245,000 would > > cause the slot to advertise F0h/00b (250W), which seems wrong. > > So for slot_power_limit_mw == 245 W we should set following values? > > slot_power_limit_mw = 239 W > value = 0xF0 > scale = 0 I think Slot Cap should never advertise more power than the slot can supply. So if the DT tells us the slot can supply 245 W, I don't think Slot Cap should advertise that it can supply 250 W. I think we should drop down to the next lower possible value, which is 239 W (value 0xEF, scale 0). I think this is what your v4 does. > > I think we should do something like this instead: > > > > scale = 0; > > if (slot_power_limit_mw >= 600*1000) { > > value = 0xFE; > > slot_power_limit_mw = 600*1000; > > } else if (slot_power_limit_mw >= 575*1000) { > > value = 0xFD; > > slot_power_limit_mw = 575*1000; > > } ... > > This is already implemented in branch: > > } else if (slot_power_limit_mw <= 600*1000) { > value = 0xF0 + (slot_power_limit_mw / 1000 - 250) / 25; > scale = 0; OK, I was thinking there was a hole here, but I guess not. I think do think it's easier to read and verify if it's structured as "the slot can supply at least X, so advertise X", as opposed to "the slot can supply X or less, so advertise Y". Bjorn