linux-kernel - Re: [RESEND PATCH v3 5/7] PCI: dwc: all: Modify dbi accessors to access data of 4/2/1 bytes

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Date:   Fri, 10 Mar 2017 18:34:45 +0530
From:   Kishon Vijay Abraham I <kishon@...com>
To:     Niklas Cassel <niklas.cassel@...s.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        <linux-pci@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-omap@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>
CC:     <nsekhar@...com>, Jingoo Han <jingoohan1@...il.com>,
        Richard Zhu <hongxing.zhu@....com>,
        Lucas Stach <l.stach@...gutronix.de>,
        Murali Karicheri <m-karicheri2@...com>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
        Jesper Nilsson <jespern@...s.com>,
        Zhou Wang <wangzhou1@...ilicon.com>,
        Gabriele Paoloni <gabriele.paoloni@...wei.com>
Subject: Re: [RESEND PATCH v3 5/7] PCI: dwc: all: Modify dbi accessors to
 access data of 4/2/1 bytes

Hi,

On Friday 10 March 2017 06:26 PM, Niklas Cassel wrote:
> On 03/10/2017 01:04 PM, Kishon Vijay Abraham I wrote:
>> Hi,
>>
>> On Thursday 09 March 2017 08:18 PM, Niklas Cassel wrote:
>>> On 03/09/2017 07:39 AM, Kishon Vijay Abraham I wrote:
>>>> Previously dbi accessors can be used to access data of size 4
>>>> bytes. But there might be situations (like accessing
>>>> MSI_MESSAGE_CONTROL in order to set/get the number of required
>>>> MSI interrupts in EP mode) where dbi accessors must
>>>> be used to access data of size 2. This is in preparation for
>>>> adding endpoint mode support to designware driver.
>>> Hello Kishon
>>>
>>> I don't really like the idea of adding an extra argument to every existing read/write.
>>> Will not a read/write of length != 32 be quite uncommon compared to
>>> a read/write of length == 32?
>>>
>>> How about adding some defines to pcie-designware.h:
>>>
>>> #define dw_pcie_writel_dbi(pci, base, reg, val) dw_pcie_write_dbi(pci, base, reg, 0x4, val)
>>> #define dw_pcie_readl_dbi(pci, base, reg) dw_pcie_read_dbi(pci, base, reg, 0x4)
>>>
>>> That way we don't have to change every existing read/write.
>>>
>>>
>>>
>>> Is there a reason why we can't just do:
>>>
>>> vial = dw_pcie_readl_dbi(pci, base, MSI_MESSAGE_CONTROL);
>> MSI_MESSAGE_CONTROL is 0x52 (MSI capability offset + 2). I'm not sure if we can
>> do a readl that crosses the alignment boundary in all platforms. The other
>> option is to readl from "MSI capability offset + 0" and extract the last 16
>> bits. I felt this is more clear since we are interested only in the
>> MSI_MESSAGE_CONTROL.
>>
>>> <shifting+masking the bits we need to get/set>
>>> dw_pcie_writel_dbi(pci, base, MSI_MESSAGE_CONTROL, val);
>>>
>>> Or are we going to be doing read/writes of length != 32 so often that
>>> you think that it's cleaner to have this abstraction?
>> it's used mainly for accessing configuration space header fields. Even the pci
>> core uses *pci_read_config_word* for accessing such fields.
> 
> I see. Adding an extra size argument is a good thing then,
> since it's consistent with the pci generic code.
> 
> However, I still think that having defines for writel/readl is a
> good thing :)

sure, having defines is fine. How about something like below (readl, readw: to
differentiate 4byte and 2 byte access?)

#define dw_pcie_readl_dbi(pci, base, reg) __dw_pcie_read_dbi(pci, base, reg, 0x4)
#define dw_pcie_readw_dbi(pci, base, reg) __dw_pcie_read_dbi(pci, base, reg, 0x2)

Thanks
Kishon