linux-kernel - Re: [PATCH v10 03/26] gunyah: Common types and error codes for Gunyah hypercalls

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Date:   Thu, 02 Mar 2023 08:18:21 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Elliot Berman" <quic_eberman@...cinc.com>,
        "Alex Elder" <alex.elder@...aro.org>,
        "Alex Elder" <elder@...aro.org>,
        "Srinivas Kandagatla" <srinivas.kandagatla@...aro.org>,
        "Prakruthi Deepak Heragu" <quic_pheragu@...cinc.com>
Cc:     "Murali Nalajala" <quic_mnalajal@...cinc.com>,
        "Trilok Soni" <quic_tsoni@...cinc.com>,
        "Srivatsa Vaddagiri" <quic_svaddagi@...cinc.com>,
        "Carl van Schaik" <quic_cvanscha@...cinc.com>,
        "Dmitry Baryshkov" <dmitry.baryshkov@...aro.org>,
        "Bjorn Andersson" <andersson@...nel.org>,
        "Konrad Dybcio" <konrad.dybcio@...aro.org>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Rob Herring" <robh+dt@...nel.org>,
        "Krzysztof Kozlowski" <krzysztof.kozlowski+dt@...aro.org>,
        "Jonathan Corbet" <corbet@....net>,
        "Bagas Sanjaya" <bagasdotme@...il.com>,
        "Catalin Marinas" <catalin.marinas@....com>,
        "Jassi Brar" <jassisinghbrar@...il.com>,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v10 03/26] gunyah: Common types and error codes for Gunyah
 hypercalls

On Thu, Mar 2, 2023, at 02:40, Elliot Berman wrote:
> On 2/23/2023 1:58 PM, Alex Elder wrote:

>>> +enum gh_error {
>>> +    GH_ERROR_OK            = 0,
>>> +    GH_ERROR_UNIMPLEMENTED        = -1,
>>> +    GH_ERROR_RETRY            = -2,
>> 
>> Do you expect this type to have a particular size?
>> Since you specify negative values, it matters, and
>> it's possible that this forces it to be a 4-byte value
>> (though I'm not sure what the rules are).  In other
>> words, UNIMPLEMENTED could conceivably have value 0xff
>> or 0xffffffff.  I'm not even sure you can tell whether
>> an enum is interpreted as signed or unsigned.
>
> I'm not a C expert, but my understanding is that enums are signed. 
> Gunyah will be returning a signed 64-bit register, however there's no 
> intention to go beyond 32 bits of error codes since we want to work on 
> 32-bit architectures.

This came up recently because gcc-13 changes the rules.

In GNU C, the enum type will have the smallest type that fits all
values, so if it contains a negative number it ends up as a signed
type (int, long or long long), but if all values are positive and at
least one of them exceeds the signed range (e.g. UINT_MAX), it is
an unsigned type. If it contains both UINT_MAX and -1, the enum
type gets changed to a signed 64-bit type in order to fit both.

Before gcc-13, the individual constants have the smallest type
(at least 'int') that fits their value, but in gcc-13 they have
the same type as the enum type itself.

     Arnd