Message-ID: <CADaLNDkuXpMVanyPD1oMOK-uvdt+vK2+UabgUxvaK9bBZDmdKg@mail.gmail.com>
Date:	Fri, 17 Apr 2015 03:00:41 -0700
From:	Duc Dang <dhdang@....com>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	Feng Kan <fkan@....com>, Arnd Bergmann <arnd@...db.de>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Liviu Dudau <Liviu.Dudau@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	Tanmay Inamdar <tinamdar@....com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	linux-arm <linux-arm-kernel@...ts.infradead.org>,
	Loc Ho <lho@....com>
Subject: Re: [PATCH v3 1/4] PCI: X-Gene: Add the APM X-Gene v1 PCIe MSI/MSIX
 termination driver

On Wed, Apr 15, 2015 at 1:16 AM, Marc Zyngier <marc.zyngier@....com> wrote:
> On Tue, 14 Apr 2015 19:20:19 +0100
> Duc Dang <dhdang@....com> wrote:
>
>> On Sat, Apr 11, 2015 at 5:06 AM, Marc Zyngier <marc.zyngier@....com>
>> wrote:
>> > On 2015-04-11 00:42, Duc Dang wrote:
>> >>
>> >> On Fri, Apr 10, 2015 at 10:20 AM, Marc Zyngier
>> >> <marc.zyngier@....com> wrote:
>> >>>
>> >>> On 09/04/15 18:05, Duc Dang wrote:
>> >>>>
>> >>>> X-Gene v1 SoC supports total 2688 MSI/MSIX vectors coalesced into
>> >>>> 16 HW IRQ lines.
>> >>>>
>> >>>> Signed-off-by: Duc Dang <dhdang@....com>
>> >>>> Signed-off-by: Tanmay Inamdar <tinamdar@....com>
>> >>>> ---
>> >>>>  drivers/pci/host/Kconfig         |   6 +
>> >>>>  drivers/pci/host/Makefile        |   1 +
>> >>>>  drivers/pci/host/pci-xgene-msi.c | 407
>> >>>> +++++++++++++++++++++++++++++++++++++++
>> >>>>  drivers/pci/host/pci-xgene.c     |  21 ++
>> >>>>  4 files changed, 435 insertions(+)
>> >>>>  create mode 100644 drivers/pci/host/pci-xgene-msi.c
>> >>>>
>> >>>> diff --git a/drivers/pci/host/Kconfig b/drivers/pci/host/Kconfig
>> >>>> index 7b892a9..c9b61fa 100644
>> >>>> --- a/drivers/pci/host/Kconfig
>> >>>> +++ b/drivers/pci/host/Kconfig
>> >>>> @@ -89,11 +89,17 @@ config PCI_XGENE
>> >>>>         depends on ARCH_XGENE
>> >>>>         depends on OF
>> >>>>         select PCIEPORTBUS
>> >>>> +       select PCI_MSI_IRQ_DOMAIN if PCI_MSI
>> >>>> +       select PCI_XGENE_MSI if PCI_MSI
>> >>>>         help
>> >>>>           Say Y here if you want internal PCI support on APM
>> >>>> X-Gene SoC. There are 5 internal PCIe ports available. Each port
>> >>>> is GEN3 capable
>> >>>>           and have varied lanes from x1 to x8.
>> >>>>
>> >>>> +config PCI_XGENE_MSI
>> >>>> +       bool "X-Gene v1 PCIe MSI feature"
>> >>>> +       depends on PCI_XGENE && PCI_MSI
>> >>>> +
>> >>>>  config PCI_LAYERSCAPE
>> >>>>         bool "Freescale Layerscape PCIe controller"
>> >>>>         depends on OF && ARM
>> >>>> diff --git a/drivers/pci/host/Makefile
>> >>>> b/drivers/pci/host/Makefile index e61d91c..f39bde3 100644
>> >>>> --- a/drivers/pci/host/Makefile
>> >>>> +++ b/drivers/pci/host/Makefile
>> >>>> @@ -11,5 +11,6 @@ obj-$(CONFIG_PCIE_SPEAR13XX) +=
>> >>>> pcie-spear13xx.o obj-$(CONFIG_PCI_KEYSTONE) += pci-keystone-dw.o
>> >>>> pci-keystone.o obj-$(CONFIG_PCIE_XILINX) += pcie-xilinx.o
>> >>>>  obj-$(CONFIG_PCI_XGENE) += pci-xgene.o
>> >>>> +obj-$(CONFIG_PCI_XGENE_MSI) += pci-xgene-msi.o
>> >>>>  obj-$(CONFIG_PCI_LAYERSCAPE) += pci-layerscape.o
>> >>>>  obj-$(CONFIG_PCI_VERSATILE) += pci-versatile.o
>> >>>> diff --git a/drivers/pci/host/pci-xgene-msi.c
>> >>>> b/drivers/pci/host/pci-xgene-msi.c
>> >>>> new file mode 100644
>> >>>> index 0000000..4f0ff42
>> >>>> --- /dev/null
>> >>>> +++ b/drivers/pci/host/pci-xgene-msi.c
>> >>>> @@ -0,0 +1,407 @@
>> >>>> +/*
>> >>>> + * APM X-Gene MSI Driver
>> >>>> + *
>> >>>> + * Copyright (c) 2014, Applied Micro Circuits Corporation
>> >>>> + * Author: Tanmay Inamdar <tinamdar@....com>
>> >>>> + *        Duc Dang <dhdang@....com>
>> >>>> + *
>> >>>> + * This program is free software; you can redistribute  it
>> >>>> and/or modify it
>> >>>> + * under  the terms of  the GNU General  Public License as
>> >>>> published by the
>> >>>> + * Free Software Foundation;  either version 2 of the  License,
>> >>>> or (at your
>> >>>> + * option) any later version.
>> >>>> + *
>> >>>> + * This program is distributed in the hope that it will be
>> >>>> useful,
>> >>>> + * but WITHOUT ANY WARRANTY; without even the implied warranty
>> >>>> of
>> >>>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>> >>>> + * GNU General Public License for more details.
>> >>>> + */
>> >>>> +#include <linux/interrupt.h>
>> >>>> +#include <linux/module.h>
>> >>>> +#include <linux/msi.h>
>> >>>> +#include <linux/of_irq.h>
>> >>>> +#include <linux/pci.h>
>> >>>> +#include <linux/platform_device.h>
>> >>>> +#include <linux/of_pci.h>
>> >>>> +
>> >>>> +#define MSI_INDEX0             0x000000
>> >>>> +#define MSI_INT0               0x800000
>> >>>> +
>> >>>> +struct xgene_msi_settings {
>> >>>> +       u32     index_per_group;
>> >>>> +       u32     irqs_per_index;
>> >>>> +       u32     nr_msi_vec;
>> >>>> +       u32     nr_hw_irqs;
>> >>>> +};
>> >>>> +
>> >>>> +struct xgene_msi {
>> >>>> +       struct device_node              *node;
>> >>>> +       struct msi_controller           mchip;
>> >>>> +       struct irq_domain               *domain;
>> >>>> +       struct xgene_msi_settings       *settings;
>> >>>> +       u32                             msi_addr_lo;
>> >>>> +       u32                             msi_addr_hi;
>> >>>
>> >>>
>> >>> I'd rather see the mailbox address directly, and only do the
>> >>> split when assigning it to the message (you seem to play all kind
>> >>> of tricks on the address anyway).
>> >>
>> >>
>> >> msi_addr_lo and msi_addr_hi store the physical base address of MSI
>> >> controller registers. I will add comment to clarify this.
>> >
>> >
>> > What I mean is that there is no point in keeping this around as a
>> > pair of 32bit variables. You'd better keep it as a single 64bit,
>> > and do the split when assigning it the the MSI message.
>>
>> Hi Marc,
>>
>> These came from device-tree (which describes 64-bit address number as
>> 2 32-bit words).
>
> ... and converted to a resource as a 64bit word, on which you apply
> {upper,lower}_32_bit(). So much for DT...
>
>> If I store them this way, I don't need CPU cycles to do the split
>> every time assigning them to the MSI message. Please let me know what
>> do you think about it.
>
> This is getting absolutely silly.
>
> How many cycles does it take to execute "lsr x1, x0, #32" on X-Gene? If
> it takes so long that it is considered to be a bottleneck, I suggest
> you go and design a better CPU (hint: the answer is probably 1 cycle
> absolutely everywhere).
>
> How often are you configuring MSIs in the face of what is happening in
> the rest of the kernel? Almost never!
>
> So, given that "never" times 1 is still never,  I'll consider that
> readability of the code trumps it anytime (I can't believe we're having
> that kind of conversation...).
>
I changed to use u64 for msi_addr and split it when composing MSI messages.
The change is in v4 of the patch set that I just posted.
>> >
>> > [...]
>> >
>> >>>> +static int xgene_msi_set_affinity(struct irq_data *irq_data,
>> >>>> +                                 const struct cpumask *mask,
>> >>>> bool force)
>> >>>> +{
>> >>>> +       struct xgene_msi *msi =
>> >>>> irq_data_get_irq_chip_data(irq_data);
>> >>>> +       unsigned int gic_irq;
>> >>>> +       int ret;
>> >>>> +
>> >>>> +       gic_irq = msi->msi_virqs[irq_data->hwirq %
>> >>>> msi->settings->nr_hw_irqs];
>> >>>> +       ret = irq_set_affinity(gic_irq, mask);
>> >>>
>> >>>
>> >>> Erm... This as the effect of moving *all* the MSIs hanging off
>> >>> this interrupt to another CPU. I'm not sure that's an acceptable
>> >>> effect... What if another MSI requires a different affinity?
>> >>
>> >>
>> >> We have 16 'real' hardware IRQs. Each of these has multiple MSIs
>> >> attached to it.
>> >> So this will move all MSIs handing off this interrupt to another
>> >> CPU; and we don't support different affinity settings for
>> >> different MSIs that are attached to the same hardware IRQ.
>> >
>> >
>> > Well, that's a significant departure from the expected behaviour.
>> > In other words, I wonder how useful this is. Could you instead
>> > reconfigure the MSI itself to hit the right CPU (assuming you don't
>> > have more than 16 CPUs and if
>> > that's only for XGene-1, this will only be 8 at most)? This would
>> > reduce your number of possible MSIs, but at least the semantics of
>> > set_afinity would be preserved.
>>
>> X-Gene-1 supports 2688 MSIs that are divided into 16 groups, each
>> group has 168 MSIs that are mapped to 1 hardware GIC IRQ (so we have
>> 16 hardware GIC IRQs for 2688 MSIs).
>
> We've already established that.
>
>> Setting affinity of single MSI to deliver it to a target CPU will move
>> all the MSIs mapped to the same GIC IRQ to that CPU as well. This is
>> not a standard behavior, but limiting the total number of MSIs will
>> cause a lot of devices to fall back to INTx (with huge performance
>> penalty) or even fail to load their driver as these devices request
>> more than 16 MSIs during driver initialization.
>
> No, I think you got it wrong. If you have 168 MSIs per GIC IRQ, and
> provided that you have 8 CPUs (XGene-1), you end up with 336 MSIs per
> CPU (having statically assigned 2 IRQs per CPU).
>
> Assuming you adopt my scheme, you still have a grand total of 336 MSIs
> that can be freely moved around without breaking any userspace
> expectations.
>
Thanks Marc. This is a very good idea.

But to  move MSIs around, I need to change MSI termination address and data
and write them to device configuration space. This may cause problems
if the device
fires an interrupt at the same time when I do the config write?

What is your opinion here?

> I think that 336 MSIs is a fair number (nowhere near the 16 you claim).
> Most platforms are doing quite well with that kind of numbers. Also,
> you don't have to allocate all the MSIs a device can possibly claim (up
> to 2048 MSI-X per device), as they are all perfectly capable of using
> less MSI without having to fallback to INTx).
>
>> I can document the limitation in affinity setting of X-Gene-1 MSI in
>> the driver to hopefully not make people surprise and hope to keep the
>> total number of supported MSI as 2688 so that we can support as many
>> cards that require MSI/MSI-X as possible.
>
> I don't think this is a valid approach. This breaks userspace (think of
> things like irqbalance), and breaks the SMP affinity model that Linux
> uses. No amount of documentation is going to solve it, so I think you
> just have to admit that the HW is mis-designed and do the best you can
> to make it work like Linux expect it to work.
>
> The alternative would to disable affinity setting altogether instead of
> introducing these horrible side effects.
>
I have it disabled (set_affinity does nothing) in my v4 patch.

> Thanks,
>
>         M.
> --
> Jazz is not dead. It just smells funny.
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