linux-kernel - Re: [PATCHv5 04/20] PCI: mobiveil: Remove the flag MSI_FLAG_MULTI_PCI

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Date:   Wed, 12 Jun 2019 12:22:41 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        "Z.q. Hou" <zhiqiang.hou@....com>
Cc:     "mark.rutland@....com" <mark.rutland@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Xiaowei Bao <xiaowei.bao@....com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "l.subrahmanya@...iveil.co.in" <l.subrahmanya@...iveil.co.in>,
        "will.deacon@....com" <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Leo Li <leoyang.li@....com>,
        "M.h. Lian" <minghuan.lian@....com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        Mingkai Hu <mingkai.hu@....com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCHv5 04/20] PCI: mobiveil: Remove the flag MSI_FLAG_MULTI_PCI_MSI

On Tue, 11 Jun 2019 18:29:49 +0100,
Marc Zyngier <marc.zyngier@....com> wrote:
> 
> On 11/06/2019 17:59, Lorenzo Pieralisi wrote:
> > On Fri, Apr 12, 2019 at 08:35:36AM +0000, Z.q. Hou wrote:
> >> From: Hou Zhiqiang <Zhiqiang.Hou@....com>
> >>
> >> The current code does not support multiple MSIs, so remove
> >> the corresponding flag from the msi_domain_info structure.
> > 
> > Please explain me what's the problem before removing multi MSI
> > support.
> 
> The reason seems to be the following code in the allocator:
> 
>         WARN_ON(nr_irqs != 1);
>         mutex_lock(&msi->lock);
> 
>         bit = find_first_zero_bit(msi->msi_irq_in_use, msi->num_of_vectors);
>         if (bit >= msi->num_of_vectors) {
>                 mutex_unlock(&msi->lock);
>                 return -ENOSPC;
>         }
> 
>         set_bit(bit, msi->msi_irq_in_use);
> 
> So instead of fixing the allocator, the author prefers disabling
> the feature. I'm not sure whether that is an acceptable outcome...

Actually, there is a much deeper issue, and the compose_msi_msg
callback gives a clue:

  phys_addr_t addr = pcie->pcie_reg_base + (data->hwirq * sizeof(int));

This thing is using a separate target address per MSI, which is the
killer argument. Bad hardware...

Thanks,

	M.

-- 
Jazz is not dead, it just smells funny.